Allow for parallel builds and saved output

The MAKEALL script cleverly runs make with the appropriate options
to use all of the cores on the system, but your average U-Boot build
can't make much use of more than a few cores.  If you happen to have
a many-core server, your builds will leave most of the system idle.

In order to make full use of such a system, we need to build multiple
targets in parallel, and this requires directing make output into
multiple directories. We add a BUILD_NBUILDS variable, which allows
users to specify how many builds to run in parallel.
When BUILD_NBUILDS is set greater than 1, we redefine BUILD_DIR for
each build to be ${BUILD_DIR}/${target}. Also, we make "./build" the
default BUILD_DIR when BUILD_NBUILDS is greater than 1.

MAKEALL now tracks which builds are still running, and when one
finishes, it starts a new build.

Once each build finishes, we run "make tidy" on its directory, to reduce
the footprint.

As a result, we are left with a build directory with all of the built
targets still there for use, which means anyone who wanted to use
MAKEALL as part of a test harness can now do so.

Signed-off-by: Andy Fleming <afleming@freescale.com>