Defaults the reserved blocks for root to 0% on read only partitions
(system, system_other, vendor, oem). It also adds support for
explicitly specifying the extfs reserved percentage via
BOARD_{SYSTEM,VENDOR,OEM,PRODUCT}IMAGE_EXTFS_RSV_PCT.
This eventually translates down to the -m option for mkfs.
Removing the reserved space can save at least 5% from the default.
dumpe2fs system:
Reserved blocks uid: 0 (user root)
Reserved blocks gid: 0 (group root)
Bug: 75975085
Test: Build, verify reserved space is changed accordingly
Change-Id: I212d82741908b636db0d658a1c4847bbaadfd5ba
5ff758799c