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>

 # This file declares functions for bare metal kernel (IE. none)

 # Copyright 2008 Yann E. MORIN

 # Licensed under the GPL v2. See COPYING in the root of this package

 CT_DoKernelTupleValues() {

     # For bare-metal, there is no kernel part in the tuple

     CT_TARGET_KERNEL=

 }

 do_kernel_get() {

     :

 }

 do_kernel_extract() {

     :

 }

 do_kernel_headers() {

     :

 }