build_image: Remove disable_read_write variable
build_image: Remove disable_read_write variable

The disable_read_write variable was just a copy of FLAGS_enable_rootfs_verification,
so to make things less confusing just use FLAGS_enable_rootfs_verification.

Signed-off-by: Geoff Levand <c3918f39022407dee6a0056ceca9b1aa80e8e4ce@infradead.org>

coreos/scripts,glevand/coreos--scripts,coreos/scripts,glevand/coreos--scripts