BuildSystem: Provide Overlay Feeds base infrastructure implementation

Problem: OpenWRT Build system provide no way to extend package from 3rd party feed.
As a result in order to manage CVE patches or small modification to a single package
we should make copy of package in feeds that we own and monitor any changes in the
package original feed to not miss upstream updates.

Solution: Include all $(PKG_DIR_NAME)/Makefile.append files as part of first
$(eval $(call BuildPackage,packagename)) call in original $(PKG_DIR_NAME)/Makefile.
Add append directories to PKG_FILE_DEPENDS in order to invalidate build when
append files changed.
Together with this change provide several useful functions and variables:

Variable $(THISDIR) -- defined in context of Makefile.append files to and points to
the directory where Makefile.append recipe. Can be used to find additional patches,
config fragments, etc.

Variable $(OVERLAY_PATCHES) -- can be used to provide additional patch files for package
sources in the following manner:

    OVERLAY_PATCHES := $(OVERLAY_PATCHES) $(THISDIR)/patches/filename.patch
    OVERLAY_PATCHES := $(OVERLAY_PATCHES) $(wildcard $(THISDIR)/patches/*.patch)

Variable $(newline) -- contains single "\n" symbol. Required to append/prepend
OpenWRT multiline variables with additional line.

Functions (append,VARNAME,value[,override]) -- used to append "value" string to variable VARNAME.
Behave like "+=" with two exceptions:
 1. Append "value" without extra space before the value
 2. The "value" expanded immediately before the assignment, while VARNAME is treated
    as recursive expand variable. Such behavior allow you to use $(THISDIR) variable
    together with "append" function. For example the following call will expand
    $(THISDIR) immediately:

    $(call append,Package/helloworld/config,$(newline)source "$(THISDIR)/Config.in")

Function (prepend,VARNAME,value[,override]) -- used to prepend "value" string to variable VARNAME.
Shares "append" function behavior regards expansion time. In order to append string
with delayed expansion time you can replace $(...) with $$(...). For exaple the
following call will delay $(Build/IncludeOverlay) expansion:

    $(call prepend,BuildPackage,$$(Build/IncludeOverlay)$(newline))

Function (remove,VARNAME,value[,override]) -- behave like "filter-out" function, but allow to
control expansion time of the passed "value".

Function (rewrite,VARNAME,value[,override]) -- generalized function that allow arbitrary change
of VARNAME variable value controlling "value" expansion time.

All this functions support optional last argument that allow to modify "override VARNAME" instead
of non-override version of VARNAME.

References: PCF-1615
Signed-off-by: Ivan Efimov <i.efimov@inango-systems.com>
Signed-off-by: Nicolai Scurtu <n.scurtu@inango-systems.com>
Signed-off-by: Vadym Omelchuk <v.omelchuk@inango-systems.com>
Signed-off-by: Vladimir Shulika <v.shulika@inango-systems.com>
Signed-off-by: Nikolai Merinov <n.merinov@inango-systems.com>

include/overlay-feed/helpers.mk

@@ -0,0 +1,22 @@
+define rewrite
+$(eval $(3) define $(1)
+$(2)
+endef)
+endef
+
+define prepend
+$(call rewrite,$(1),$(2)$(value $(1)),$(3))
+endef
+
+define append
+$(call rewrite,$(1),$(value $(1))$(2),$(3))
+endef
+
+define remove
+$(call rewrite,$(1),$(filter-out $(2),$(value $(1))),$(3))
+endef
+
+define newline
+
+
+endef

include/overlay-feed/package.mk

@@ -0,0 +1,27 @@
+include $(INCLUDE_DIR)/overlay-feed/helpers.mk
+
+# All patches which paths added into OVERLAY_PATCHES
+# will be applied on package build dir after Prepare step
+# of main package
+OVERLAY_PATCHES ?=
+
+define Overlay/Prepare/Patch
+    $(foreach pf,$(filter %.patch,$(OVERLAY_PATCHES)), \
+            patch -d "$(PKG_BUILD_DIR)" -p1 < $(pf) $$(newline) \
+    )
+endef
+Hooks/Prepare/Post += Overlay/Prepare/Patch
+
+OVERLAY_MAKEFILE_APPENDS := $(foreach feed,$(shell $(TOPDIR)/scripts/feeds list -n 2>/dev/null),$(shell find -L $(TOPDIR)/feeds/$(feed)/ -path '*/$(PKG_DIR_NAME)/Makefile.append' | sort))
+PKG_FILE_DEPENDS += $(foreach mk,$(OVERLAY_MAKEFILE_APPENDS), $(dir $(mk)))
+
+define Build/IncludeOverlay
+    # Clean out Build/IncludeOverlay to prevent recursive expansion
+    $(eval Build/IncludeOverlay=)
+    $(foreach mk,$(OVERLAY_MAKEFILE_APPENDS), \
+        $(eval THISDIR := $(dir $(mk))) \
+        $(eval -include $(mk)) \
+    )
+endef
+
+$(call prepend,BuildPackage,$$(Build/IncludeOverlay)$(newline))

include/package.mk

@@ -379,3 +379,5 @@ dist:
 
 distcheck:
 	$(Build/DistCheck)
+
+include $(INCLUDE_DIR)/overlay-feed/package.mk