scripts: update docstring for xbbmake
git: add .gitignore
benchmark: add initial benchmark .spec files

benchmark.armel-base.beagleboard.spec and benchmark.armhf-base.beagleboard.spec
provide a simple and highly optimizable binary that can be used to
benchmark different compile flags to demonstrate how ubuild can
ease the optimization phase (if we were to write many .spec files
with different CFLAGS combinations).