Impact: Bugfix, avoids kernels which build but panic on boot

Fix a bug in decompress.c : only scanned until the first
non-configured compressor (with disastrous result especially if that
was gzip.)

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

Instead of failing to identify a compressed image with a decompressor
that we don't have compiled in, identify it and fail with a
comprehensible panic message.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

Centralize the compression format detection to a common routine in the
lib directory, and use it for both initramfs and initrd.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>