Decompress and install the edk2 firmware blobs as part of "make install",
unless blob installation was disabled with configure's "--disable-blobs"
option.

Additionally, decompress the blobs as a pre-requisite for building softmmu
binaries -- this is helpful for both "make check" and other ad-hoc tests
one might want to run in the build directory.

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>