Make XENFIRMWAREDIR a subdir of libexecdir

Make XENFIRMWAREDIR a subdir of libexecdir

Put the firmware files below libexecdir. This is essentially just a new
name for the existing path. It has the benefit that it can be configured
via --libexecdir= if required.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
[ ijc -- ran autogen.sh as requested ]