xlat: override values of KEY_RFKILL and KEY_BRIGHTNESS_MIN constants

xlat: override values of KEY_RFKILL and KEY_BRIGHTNESS_MIN constants

KEY_RFKILL and KEY_NUMERIC_A constants were introduced by Linux kernel
commits v2.6.33~17^2~2 and v4.1-rc1~150^2~1^10~6, respectively.
Apparently, RHEL6 introduced an alternative KEY_RFKILL constant
with the same value as upstream KEY_NUMERIC_A.

KEY_BRIGHTNESS_MIN and KEY_BRIGHTNESS_MAX constants were introduced
by Linux kernel commit v3.16-rc1~30^2~6^2~1^2~7.
Apparently, RHEL7 introduced an alternative KEY_BRIGHTNESS_MIN constant
with the same value as upstream KEY_BRIGHTNESS_MAX.

Downstream vendors are *not* allowed to do this.
Ignore the system value of KEY_RFKILL and KEY_BRIGHTNESS_MIN.

* xlat/evdev_keycode.in (KEY_RFKILL, KEY_BRIGHTNESS_MIN): Undefine.