vendorcode/amd/agesa/fam15tn: Build as a static library

Up until now, we were building AGESA by specifying each AGESA source
file and adding it to the list of romstage and ramstage source files.
As a result, we were compiling each AGESA source twice, despite the
fact that it does not depend on the stage we're in.

Since AGESA is stage-independent, we can build it just once, and use
the resulting static library in both rom and ram stages.

We still keep the practice of specifying every single AGESA directory
as an include dir and adding the AGESA CFLAGS to our global CFLAGS;
this is needed due to the way AGESA builds.

Change-Id: I9b23264129d1c08cb67cabc31d15a68d43ed7624
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>