hush: Pull out U-Boot prompt display and read functionality

Pull the code which displays U-Boot prompt and reads the command line
into a separate function. No functional change.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>