payloads: make build system integration work again

Payloads using Kconfig get confused by coreboot Kconfig
configuration in environment variables. Prune them.

Change-Id: I63da2af0a15dca35d70cd65b2f74a1564aab9483
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/5710
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Idwer Vollering <vidwer@gmail.com>
Makefile.inc: Make clang once again a valid toolchain

'prove' that clang is supported (to some extent).

Change-Id: I181f4910ba64ab9746e7ac94aa79da23cdd41dad
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/5709
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>
Tested-by: build bot (Jenkins)
Makefile: Don't use llvm-mc for the moment

The LLVM integrated assembler has some deficiencies in support for
building AGESA. See:

LLVM PR18918 - [RFE]: Missing altmacro support in integrated assembler

Disable llvm-mc for the moment until these have been addressed fully
upstream.

Change-Id: Id4131d1de04d01c0bec284f976f0ba9662b950ab
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/5711
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>
Tested-by: build bot (Jenkins)