gdb: enable tui support for target package

Add a configuration option to compile the gdb target package with the
--enable-tui switch.

This is done pretty much in the same way as in commit 2474fb0b ("host-gdb:
enable terminal user interface support"), but for the gdb package on target.

This makes sense only when a full debugger is installed on target,
not for a gdbserver.

[Thomas: remove "default n" since this is the default, adjust the
prompt of the option, and rewrap the help text.]

Signed-off-by: Vincent Stehlé <vincent.stehle@laposte.net>
Cc: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>