Makefile: Add EXTRA_CFLAGS variable

Distributions packaging dtc may need to set extra flags. Currently when
they do that it overrides the ones set by the makefile. This is
particularly problematic when compiling without yaml, as the yaml
detection is ignored.

 ld: dtc.o: in function `main':
 dtc.c:(.text.startup+0x718): undefined reference to `dt_to_yaml'

This patch provides a EXTRA_CFLAGS variable that is added to the list of
CFLAGS, and can be set on the command line when packaging.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Message-Id: <20190722030244.9580-1-joel@jms.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>