Avoid repeated building of erofs image

The image size of erofs is only affected by the content of the input
file, so it doesn't care about the input image_size. In addition, the
build time of erofs image is longer than ext4 due to compression. Only
execute mkfs once to reduce unnecessary time consumption.

Bug: 199248596
Test: erofs image build faster than before
Signed-off-by: Huang Jianan <huangjianan@oppo.com>
Change-Id: I23a4e640d5066485de3fea0f808998045e716d1f