ci: testbed: labgrid: restore lcm_data cleanup dropped by FEAT-29 flow

The FEAT-29 update_prpl conversion switched both the Freedom (wnc-freedom)
and OSPv2 (mxl25641-hdk-6) eMMC flash flows to erase only the rootfs_data
overlay, and in doing so dropped the lcm_data partition cleanup that
PPW-718 had introduced. The persistent /lcm (lcm_data) partition is not
cleared between CI runs, so stale containers from a previous test survive
the reboot/flash procedure and make the LCM cram suite non-idempotent.

So lets restore the cleanup by erasing the first 0x400 blocks (the eMMC
erase-group size, i.e. just the ext4 filesystem metadata, not the whole
multi-GiB partition) of lcm_data right after the rootfs_data erase, so
every refresh starts from a clean /lcm. The offsets are taken from each
board's current prpl Standard Flash Layout GPT, verified live in U-Boot:

  Freedom (IPQ9574#):
   28	0x00134022	0x00534021	"lcm_data"
   29	0x00534022	0x00574021	"rootfs_data"

  OSPv2 (Lightning #):
   14	0x00086c00	0x00096bff	"rootfs_data"
   16	0x000a6c00	0x006a6bff	"lcm_data"

Fixes: CI-2
References: 78b11d2d5b ("ci: testbed: labgrid: flash OSPv2 and Freedom via FEAT-29 update_prpl")
References: PPW-718
Assisted-by: claude:opus-4-8:1m-context
Signed-off-by: Petr Štetiar <petr.stetiar@prplfoundation.org>

.testbed/labgrid/default.yaml

@@ -131,7 +131,8 @@ targets:
           - setenv img_smd
           - setenv update_rescue_bank yes
           - mmc dev 0
-          - mmc erase 0x534022 0x40000
+          - mmc erase 0x534022 0x40000 # rootfs_data
+          - mmc erase 0x134022 0x400 # lcm_data
           - run update_prpl
         boot_command: run bootcmd
       UBootStrategy: {}
@@ -173,7 +174,8 @@ targets:
           - setenv img_smd
           - setenv update_rescue_bank yes
           - mmc dev 0
-          - mmc erase 0x86c00 0x10000
+          - mmc erase 0x86c00 0x10000 # rootfs_data
+          - mmc erase 0xa6c00 0x400 # lcm_data
           - run update_prpl
         boot_command: run bootcmd
       UBootStrategy: {}

.testbed/labgrid/testbed-02.yaml

@@ -165,7 +165,8 @@ targets:
           - setenv img_smd
           - setenv update_rescue_bank yes
           - mmc dev 0
-          - mmc erase 0x534022 0x40000
+          - mmc erase 0x534022 0x40000 # rootfs_data
+          - mmc erase 0x134022 0x400 # lcm_data
           - run update_prpl
         boot_command: run bootcmd
       UBootStrategy: {}
@@ -207,7 +208,8 @@ targets:
           - setenv img_smd
           - setenv update_rescue_bank yes
           - mmc dev 0
-          - mmc erase 0x86c00 0x10000
+          - mmc erase 0x86c00 0x10000 # rootfs_data
+          - mmc erase 0xa6c00 0x400 # lcm_data
           - run update_prpl
         boot_command: run bootcmd
       UBootStrategy: {}