bzip2/lzma: don't leave empty files around on failure

Impact: Bugfix, silent build failures

Fix a bug in gen_initramfs_list.sh: in case of failure, it left an
empty output file behind, messing up the next make.

Signed-off-by: Alain Knaff <alain@knaff.lu>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>