Makefile: allow overriding CFLAGS on the command line

When a Makefile variable is set on the make command line, all
Makefile-internal assignments to that very variable are _ignored_.
Since we add quite some essential values to CFLAGS internally,
specifying some CFLAGS on the command line will usually break the
build (and not fix any include file problems you hoped to overcome
with that).
Somewhat against intuition GNU make provides the "override" directive
to change this behavior; with that assignments in the Makefile get
_appended_ to the value given on the command line. [1]

Change any internal assignments to use that directive, so that a user
can use:
$ make CFLAGS=/path/to/my/include/dir
to teach kvmtool about non-standard header file locations (helpful
for cross-compilation) or to tweak other compiler options.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>

[1] https://www.gnu.org/software/make/manual/html_node/Override-Directive.html


Signed-off-by: Will Deacon <will.deacon@arm.com>