kbuild: allow compressors (gzip, bzip2, lzma) to take multiple inputs

Allow the compression commands in Kbuild (i.e. gzip, bzip2, lzma) to
take multiple input files and emit the concatenated compressed
output.  This avoids an intermediate step when a kernel image is built
from multiple components, such as the relocatable x86-32 kernel.

Sam Ravnborg integrated the bin_size script into the Makefile.

[ Impact: new build feature, not yet used ]

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>