gdb: Various fixups
gdb: Various fixups

* Apply proper cross-prefix to cross-gdb
* Enable TUI
* Use curses library instead of termcap
* Drop unneeded redefinition of ${S}
* Drop unneeded SUBDIRS argument to make

Signed-off-by: Esben Haabendal <c5ba3f1141bb30a64dd6c7e6726a53a85adc9442@prevas.dk>

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