tools/mkrpm: allow custom rpm package name

tools/mkrpm: allow custom rpm package name

Even if xen is configured and compiled with different --prefix= so that
it operates entirely below $prefix, the resulting package from 'make
rpmball' is always called "xen.rpm".

Use an environment name to give a different name.
This can be used like this:

suffix=-bugN
prefix=/opx/xen/staging${suffix}
./configure --prefix=${prefix}
make rpmball PKG_SUFFIX=${suffix}

The result will be "xen-bugN.rpm" instead of "xen.rpm". The benefit is that
many xen${suffix}.rpm packages can be installed at the same time.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>