package_rpm: Allow compression mode override

package_rpm: Allow compression mode override

Commit 4a4d5f78a6962dda5f63e9891825c80a8a87bf66 ("package_rpm: use zstd
instead of xz") changed the rpm package compressor from 'xz' to 'zstd'
which results in decompression failure with BusyBox-provided 'rpm2cpio'
applet and 'rpm' applet when given the '-i' (Install package) option:

  rpm2cpio: no gzip/bzip2/xz magic

Introduce a variable which makes it possible to use a different
compression mode, making it possible to override the default value for
example like

  RPMBUILD_COMPMODE = "${@'w6T%d.xzdio' % int(d.getVar('XZ_THREADS'))}"

to enable rpm decompression without including the full rpm package in
the resulting root filesystem.

Signed-off-by: Niko Mauno <niko.mauno@vaisala.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>