spl: fat/fs: Add control to build FS EXT4 in SPL

CONFIG_SPL_FS_EXT4 can be used to include/exclude the FS EXT4 from
SPL build. Excluding the FS EXT4 from SPL build can help to save 20KiB
memory.

Signed-off-by: Tien Fong Chee <tien.fong.chee@intel.com>
Reviewed-by: Tom Rini <trini@konsulko.com>