init: allow using the recovery image for 1st stage init

For A/B devices, the recovery image is typically provided as a ramdisk
paired with boot.img.  Instead of repartitioning these devices to have
a separate recovery partition and have the new 1st stage ramdisk
paired with the boot.img, this allows setting
androidboot.force_normal_boot=1 on the kernel command line to have init
in recovery boot directly to a normal boot.

This requires /system_recovery_mount to be added in the DT fstab for
the system partition.

Bug: 114062208
Test: hikey boots
Change-Id: I1e331b5cca91152d20ca92549779ab41aa9cd431