Fix generic ramdisk build failures on dedicated recovery partitions

For targets with BoardUsesRecoveryAsBoot set to true, binaries in
generic_ramdisk are installed to $OUT/recovery/root/first_stage_ramdisk
This path is not on generic_ramdisk.mk 's allowlist, hence build fails.
To fix the failure, add $OUT/recovery/root/first_stage_ramdisk/system to
allow list.

Test: th, lunch eos-userdebug(a dedicated recovery partition target) and build.

Bug: 265477065
Change-Id: I89cf229adc028a7e5de632b9a3188ecb411de735