Add --firmwarepath to configure

Add a firmware path config option to configure.  Multiple directories
are accepted, with the usual colon as separator.  Default value is
${prefix}/share/qemu-firmware.  The path is searched in addition to the
current search path (typically ${prefix}/share/qemu).

This prepares qemu for the planned split of the prebuilt firmware blobs
into a separate project.

Distributions can also use this to get rid of the firmware symlink farm
and add -- for example -- /usr/share/seabios to the firmware path
instead.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20170914114236.25343-3-kraxel@redhat.com