Support generate patches for all specfiles

Support generate patches for all specfiles

If specify one specfile when using gbs export, gbs will only generate patches
for this specfile. In multiply specfiles case, it will make other packages build
failed. So remove specify specfile when gbs export.

Fixes: #1276

Change-Id: I6f667a7545b2d9e9e51c6c242e7961a2dd2b390e
Signed-off-by: Lin Yang <lin.a.yang@intel.com>
Reviewed-on: https://otctools.jf.intel.com/review/6284
Tested-by: OTC Tools Tester <ed.bartosh@linux.intel.com>
Reviewed-by: Hasan Wan <hasan.wan@intel.com>
Reviewed-by: Eduard Bartosh <eduard.bartosh@intel.com>