build_image: default extfs reserved size to 0 on RO partitions.

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