nvidia/cbootimage: avoid upstream's build system

It brings in useless dependencies, a weird autotools
configuration, and tons of pain everywhere.

Instead just build things ourselves.

Change-Id: I67f06e711cb9dcd594363bc1a4f99d3273074549
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/6986
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>