Convert CONFIG LIB_HW_RAND to Kconfig

config_fallbacks.h had some logic to automatically select
LIB_RAND if RANDOM_UUID or CMD_UUID were set if LIB_HW_RAND wasn't
already selected.  By migrating LIB_HW_RAND to Kconfig, we can
remove this check from config_fallbacks.h and put it into Kconfig

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
[trini: Turn into a choice, add NET_RANDOM_ETHADDR]
Signed-off-by: Tom Rini <trini@konsulko.com>