sign_rpm.bbclass: force rpm serial signing

sign_rpm.bbclass: force rpm serial signing

Newer versions of gpg (at least 2.1.5 and 2.2.1) have issues when signing occurs in parallel
so (unfortunately) the signing must be done serially. Once the upstream problem is fixed,
this patch must be reverted, otherwise we loose all the intrinsic parallelism from
bitbake.

[YOCTO #12022]

Signed-off-by: Leonardo Sandoval <leonardo.sandoval.gonzalez@linux.intel.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>