MAKEALL: Automatically use parallel builds

Add logic to the MAKEALL script to determine the number of CPU cores
on the system, and run a parallel build if there is more than one.
Usually this significantrly accelerates builds.

Allow to manually adjust the number of parallel make jobs by using
the "BUILD_NCPUS" environment variable.

Signed-off-by: Wolfgang Denk <wd@denx.de>