ffboot v2: slot inventory, verified atomic env flips, target probe

-l inventories every bootable partition (SD, eMMC slots 1/2, legacy
p4) as machine-parsable key=value lines: firmware type
(forgefirm/factory), version, kernel presence, booted/next markers,
plus the saved-env selection - the shared probe for the installer and
the forgectrl update manager.

Boot switching now writes all four selection variables (mmcdev,
mmchwpart, mmcpart, mmcroot) in one fw_setenv -s transaction and
read-back verifies, falling back from the libubootenv script format
to the classic u-boot-tools format to per-variable writes - the same
script works on factory firmware and ForgeFIRM. mmchwpart was
previously never set and a mixed env could survive a mid-flip power
cut. Switch targets must pass a content probe (rootfs mounts, kernel
present) unless -f. -e picks the newest factory slot by probing,
excluding slots occupied by ForgeFIRM.

The new ffboot recipe installs it as /usr/sbin/ffboot with
/etc/fw_env.config (factory-identical redundant env layout at eMMC
0x80000/0x82000) - previously the image shipped fw_setenv with no
config. Added to forgefirm-image.

meta-forgefirm/recipes-forgefirm/ffboot/files/fw_env.config

@@ -0,0 +1,7 @@
+# Configuration for fw_printenv/fw_setenv.
+# The factory U-Boot saved environment: redundant 8 KiB copies in the
+# eMMC user area (matches the factory firmware's own fw_env config and
+# CONFIG_ENV_OFFSET/CONFIG_ENV_OFFSET_REDUND in the bootloader).
+# Block device	Device offset	Env. size
+/dev/mmcblk2	0x80000	0x2000
+/dev/mmcblk2	0x82000	0x2000