Enable /cache mount point

Non-A/B devices using AOSP OTA flow will mount /cache in the
fstab file. Without setting BOARD_CACHEIMAGE_FILE_SYSTEM_TYPE,
/cache will be a symlink to /data/cache which introduces the
failure of `mount_all /vendor/etc/fstab.{ro.hardware}`. This is
because all devices laucned in P need to switch to "system-as-root".

This CL sets board config to create /cache directory in rootfs
(system-as-root GSI image). Note that A/B devices doesn't mount
or use /cache so leaving an empty /cache in rootfs has no harm.

Bug: 78485405
Test: Boot GSI on walleye
Change-Id: Ic260d2917cc64c9497f5f60ea11303e953e80efd
Merged-In: Ic260d2917cc64c9497f5f60ea11303e953e80efd