Add build flags for legacy EROFS support.

EROFS images by default will only work on newer kernels. Old versions of
5.4 and 4.19 need an extra flag to mkfs.erofs.

Add BUILD_EROFS_USE_LEGACY_COMPRESSION to force this flag.

Bug: 222373106
Test: build with BUILD_EROFS_USE_LEGACY_COMPRESSION enabled
Change-Id: I3818e14356beaa11073c6b9fea6a6ad999bba4aa