[coreboot] [PATCH] Build warnings

[Myles Watson]

This patch removes these warnings:
Makefile:435: warning: overriding commands for target `src/lib/memset.o'

And replaces these debug messages:
partobj dir 0 parent <__main__.partobj instance at 0x7f1e846a7ab8>
part pci_domain
with:
partobj dir 0 parent northbridge_amd_amdk8_root_complex_dev2 part pci_domain

The warnings come about because some object files are used INITOBJECTS
and OBJECTS.  They get written to the makefile twice.  I factored out
a function that writes the rules, then only write out the rules that
haven't been written.

The debugging messages is personal taste, so if someone really objects
I'll leave that part out.

Abuild tested.  The makefiles are identical except for the missing duplicates.

Signed-off-by: Myles Watson

Thanks,
Myles