Fix error if ramdisk is minigzip compression

This change intends to fix if ramdisk is not "lz4" compression.
Legacy is "minigzip" compression.

If not lz4, the following error will happen when exec build_super_image.py:
  Unable to get boot image build props: Failed to run command '['lz4', '-d', '/tmp/boot_omdZZ8.img/ramdisk', '/tmp/boot_omdZZ8.img/uncompressed_ramdisk']' (exit code 44):
  Error 44 : Unrecognized header : file cannot be decoded

Change-Id: I71248387bbeecbf184e0c24e6346c235d728518e
Signed-off-by: jiajia tang <tangjiajia@xiaomi.com>