ui/ncurses: Reduce unnecessary calls to redrawwin

ui/ncurses: Reduce unnecessary calls to redrawwin

All current *_post() methods in ui/ncurses call redrawwin() and
wrefresh() together. wrefresh() updates any lines on the screen that
have been marked as changed or invalid. However redrawwin() marks the
entire screen as invalid unconditionally. We can reduce the amount of
data written to the screen by avoiding calls to redrawwin().

Screen transitions are the primary use case of redrawwin(), where the
whole screen must be invalidated to avoid stale data remaining on
screen. All other 'in-screen' updates such as changes to widgets or
changing focus do not require a call to redrawwin(). The most noticeable
performance improvement is in nc-menu, which makes an unnecssary call to
redrawwin() after every addition to the boot option menu.

eg. The number of bytes written to STDOUT in the main menu:
	# Boot options	| Before | After
	--------------------------------
		8	| 5488   | 1149
		133	| 422454 | 4652

Signed-off-by: Samuel Mendoza-Jonas <sam.mj@au1.ibm.com>