AOSP: erofs-utils: add fs_config support

So that mkfs can directly generate images with fs_config.
All code for AOSP is wrapped up with WITH_ANDROID macro.

Link: https://lore.kernel.org/r/20200929051302.3324-1-hsiangkao@aol.com
Acked-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <hsiangkao@aol.com>