kbuild: deb-pkg: add KDEB_SOURCE_COMPRESS to specify source compression

Add KDEB_SOURCE_COMPRESS to specify the compression for the orig and
debian tarballs. (cf. the existing KDEB_COMPRESS is used to specify
the compression for binary packages.)

Supported algorithms are gzip, bzip2, lzma, and xz, all of which are
supported by dpkg-source.

The current default is gzip. You can change it via the environment
variable, for example, 'KDEB_SOURCE_COMPRESS=xz make deb-pkg'.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>