Remove CONFIG_TARGET_MULTI_PROFILE=y and
CONFIG_TARGET_PER_DEVICE_ROOTFS=y that were hardcoded in genconfig and
genconfig_min. Since feeds/targets commit 295d1566 (treewide: Add
MULTI_PROFILE and PER_DEVICE_ROOTFS config, 2021-08-05), the options are
added in feeds/targets.
Also bring the unmaintainable copy-paste of genconfig that is
genconfig_min in sync with genconfig with regards target-specific config
options.