meta: introduce KCONFIG_CONFIG_ENABLE_MENUCONFIG

Currently, uboot do_menuconfig task is breaking when UBOOT_CONFIG is
chosen rather than UBOOT_MACHINE, it simply fails with the following
errors:
| make: *** No rule to make target 'menuconfig'. Stio.
| Command failed.
| Press any key to continue...

this is due to the work directory of do_menuconfig is set to ${B} but
not ${B}/$config.

We should distinguish two situations:
1) When there is only one config item in UBOOT_CONFIG, do_menuconfig
   should work just like how it works for UBOOT_MACHINE.
2) When there are multiple config items in UBOOT_CONFIG, do_menuconfig
   should print out some information saying it's not supported other
   than just failing.

This patch mainly aims to fix that by introducing a extra variable
KCONFIG_CONFIG_ENABLE_MENUCONFIG, it would be set to 'false' for
situation 2), and when it's set to 'true', then set
KCONFIG_CONFIG_ROOTDIR correctly in uboot-config.bbclass to let
do_menuconfig task work.

DEVTOOL_DISABLE_MENUCONFIG could be replaced by this new variable
KCONFIG_CONFIG_ENABLE_MENUCONFIG.

Signed-off-by: Ming Liu <liu.ming50@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>