Add 'build', 'build-*', and 'install' as phony targets to the top level
Makefile.
I tend to do everything in tree with the build in a directory called
'build'. This interfered with the target 'build', making it think the
target was up to date, and stopping scripts/crosstool-NG.sh from being
regenerated.
Added 'install' as I often set the prefix to $PWD/install.
Signed-off-by: Michael Hope <michael.hope@linaro.org>