Add configure --with-extra-cflags-*

Since commit 338c4375153e39b95ddc82f2ed95f85dd73e0245 ("tools: pass
EXTRA_CFLAGS via environment") it was possible to specify additional
CFLAGS in the environment. That commit gives a good explanation how that
is useful.

Now that we have configure move the knobs from the middle of the source
code to a more prominent place. If the variables are set once via
configure it is possible to rerun make from another shell which may not
have these environment variables. That can happen if one tries to resume
a failed build with either doing chroot into a rpmbuild tree or from
another xterm.

This change adds the configure options --with-extra-cflags-tools=,
--with-extra-cflags-qemu-traditional and
--with-extra-cflags-qemu-upstream which will assign their values to the
existing make variables.

A new knob "skip_extra_cflags" in Rules.mk is recognized to turn off the
assignment to CFLAGS. This is required to build the firmware.

Please rerun autogen.sh after applying this patch.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>