perf: add perf-tui feature

perf: add perf-tui feature

Add a new feature named 'perf-tui'.  Adding this into the
PERF_FEATURES variable in perf.inc will enable the perf TUI (Text-base
UI) user interface on a target, which adds libnewt and turns on the
perf text UI options in perf, if perf is included in an image.

If 'perf-tui' isn't named as a feature (the default), the perf TUI
will be disabled and unavailable.

(From OE-Core rev: 4fd8a550886f02189e4ed127d0a2f16e92f8474c)

Signed-off-by: Tom Zanussi <tom.zanussi@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>