Add UCB RISCV support for architecture, soc, and emulation mainboard..

Works in the RISCV version of QEMU.

Note that the lzmadecode is so unclean that it needs a lot of work.
A cleanup is in progress.

We decided in Prague to do this as one thing, because it forms a nice case study
of the bare minimum you need to add to get a new architecture going in qemu.

Change-Id: If5af15c3a70733d219973e0d032746f8ab027e4d
Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-on: http://review.coreboot.org/7584
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins)

toolchain.inc

@@ -57,6 +57,7 @@ ARCHDIR-i386    := x86
 ARCHDIR-x86_32  := x86
 ARCHDIR-arm     := arm
 ARCHDIR-arm64   := arm64
+ARCHDIR-riscv   := riscv
 
 CFLAGS_arm      := -mno-unaligned-access -ffunction-sections -fdata-sections