Makefile: use phony targets

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>

     1.1 --- a/Makefile.in	Tue Oct 04 10:30:54 2011 +1300
     1.2 +++ b/Makefile.in	Thu Sep 29 12:27:54 2011 +1300
     1.3 @@ -347,3 +347,5 @@
     1.4  endif # Not --local
     1.5  
     1.6  endif # No extra MAKEFLAGS were added
     1.7 +
     1.8 +.PHONY: build $(patsubst %,build-%,$(TARGETS)) install