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>

 diff -dur ELFkickers.orig/sstrip/Makefile ELFkickers/sstrip/Makefile

 --- ELFkickers.orig/sstrip/Makefile	2001-03-24 12:58:27.000000000 +0100

 +++ ELFkickers/sstrip/Makefile	2007-05-31 21:17:18.000000000 +0200

 @@ -1,7 +1,7 @@

  #  Makefile for sstrip

  sstrip: sstrip.c

 -	gcc -ggdb -Wall -W -o sstrip sstrip.c

 +	$(CC) -Wall -W -o sstrip sstrip.c

  clean:

  	rm -f sstrip