Documentation: add start of documentation of the build system

Change-Id: Ic4a4b4d71852bfe0b1fc52373e88d0a53b145844
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: https://review.coreboot.org/16150
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
1 file changed