kbuild: Fix help text not displayed in choice option.

Help text under choice menu is never displayed because it does not have
symbol name associated with it, however many kconfigs have help text
under choice, assuming that it will be displayed when user selects help.
for example in Kconfig if we have:
choice
        prompt "Choice"
        ---help---
           HELP TEXT ...

config A
        bool "A"

config B
        bool "B"

endchoice

Without this patch "HELP TEXT" is not displayed when user selects help
option when "Choice" is highlighted from menuconfig or xconfig or
gconfig.

This patch changes the logic in menu_get_ext_help to display help for
cases which dont have symbol names like choice.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Reviewed-by: Stuart Menefy <stuart.menefy@st.com>
Reviewed-by: Arnaud Lacombe <lacombar@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Michal Marek <mmarek@suse.cz>