gdb: reword prompt for host TUI option

There is no need to name the option "GDB TUI support", since this
option is already visible "below" GDB in menuconfig/xconfig. Naming it
"TUI support" is therefore sufficient.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>