rainier_defconfig: Add multi-target support

Through OpenBMC and PLDM file.io, Power10 will support multiple MRW
definitions as part of one driver.

In order to achieve this in a staged approach, this commit introduces a
new 'openpower-pnor-p10' package that still creates the expected
pnor-related artifacts and intermediate files, while also creating the
individual files for each of the PNOR partitions (PAYLOAD, BOOTKERNEL)
inside the images/mmc directory. Each of those files will be suffixed
with ".P10" for target-agnostic files, or "uppercase-xml-name" (i.e.,
RAINIER_2U_XML) for target-specific files.

This patch also creates packages rainier-2u-xml and rainier-4u-xml(based
on machine-xml) that are both built and included in rainier_defconfig,
with rainier_2u_xml also serving as the 'legacy' pnor for smooth
transitions purposes.

Several other packages makefiles had to be reworked, but that should
allow both legacy openpower-pnor as well as the new mmc variant to build.

Signed-off-by: Klaus Heinrich Kiwi <klaus@linux.vnet.ibm.com>
diff --git a/openpower/package/openpower-pnor/openpower-pnor.mk b/openpower/package/openpower-pnor/openpower-pnor.mk
index fa1ac55..9e61417 100644
--- a/openpower/package/openpower-pnor/openpower-pnor.mk
+++ b/openpower/package/openpower-pnor/openpower-pnor.mk
@@ -231,4 +231,4 @@
 
 $(eval $(generic-package))
 # Generate openPOWER pnor version string by combining subpackage version string files
-$(eval $(call OPENPOWER_VERSION,$(OPENPOWER_PNOR)))
+$(eval $(OPENPOWER_VERSION))