Setting DFLT_XKB_CONFIG_ROOT only if not defined.

Now it is possible to set DFLT_XKB_CONFIG_ROOT from
devices mkspecs.

Change-Id: I84fd74dcf77335d5c6a294b9988c2a6a80405d8d
Reviewed-by: Leon Anavi <leon@anavi.org>
Reviewed-by: Tomasz Olszak <olszak.tomasz@gmail.com>
Removed old unsused devices/linux-g++-tizen-armv7l spec.

Change-Id: I339d92990575cba3568caae8a5e5f70d0fd0089c
Reviewed-by: Leon Anavi <leon@anavi.org>
Reviewed-by: Tomasz Olszak <olszak.tomasz@gmail.com>