busybox: make sure architecture flags are used at link time

When compiling Busybox, according to readelf -A, all object files were
properly compiled according to the select architecture (-march=armv4t
for example), but the final busybox binary could be of a different
architecture (ARMv5t even if ARMv4t was selected).

This patch changes the way we configure/compile Busybox so that our
CFLAGS aren't anymore passed through the make EXTRA_CFLAGS variable,
but through the .config CONFIG_EXTRA_CFLAGS variable. Unfortunately,
those variables don't have exactly the same semantic for the Busybox
build system.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>