sm6250-common: Add common config.fs AID/fs configuration file

Common config.fs/$TARGET_FS_CONFIG_GEN AID/fs configuration
file for all chipset inheriting from base.mk.

Since legacy device-specific android_filesystem_config.h and
config.fs do not co-exist, temporarily include config.fs only
if legacy file is absent.  Once all legacy, device-specific
files have been deleted, config.fs can be made unconditional.

Change-Id: Ie100d0cd9db808b82d99be7139f0709390c7f3a2