Makefile: Respect manual changes to .config

The .config file may be manually edited or copied from another location.
Since genconfig.py is responsible for generating auto.conf (the Makefile
fragment) and autoconf.h (the C header) from .config, it must be run any
time .config changes, not just when running menuconfig.

Fixes: 662e631c

 ("Makefile: Add initial kconfig support for each platform")
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Tested-by: Anup Patel <anup@brainfault.org>