scripts/makefile-getvar: add script to get values from Makefiles

There is often a need to extract a value from a Makefile, and standard
GNU Make doesn't provide a way to do this.  This script lets you access
values from Makefiles directly:

$ makefile-getvar curl/tests/server/Makefile noinst_PROGRAMS
getpart resolve rtspd sockfilt sws tftpd fake_ntlm socksd disabled mqttd

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>