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>
Reviewed-on: http://review.coreboot.org/5430
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Idwer Vollering <vidwer@gmail.com>
southbridge/amd/agesa/hudson: Clean up AGESA #includes

Just like in commit
* 1d87dac hp/pavilion_m6_1035dx: Sanitize #includes

Include AGESA headers specifying the path relative to AGESA_ROOT. The
path is specified relative to AGESA_ROOT as opposed to src/ since this
code may include headers from different AGESA families, depending on
the board.

Change-Id: Ide38cc34e207a8b617d1d319fd9c17a785f55833
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-on: http://review.coreboot.org/5423
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Idwer Vollering <vidwer@gmail.com>