a4m072: support for SHOW_BOOT_PROGRESS feature using LED display

a4m072: support for SHOW_BOOT_PROGRESS feature using LED display

This patch adds support for displaying boot progress codes on a4m072 board
using LED display. As we can display only one symbol at any time on the hardware
(two symbols with blinking) we can't display progress codes directly and have
to map them to 2-symbol codes.
We use the following mapping on the a4m972 board:
 [1, 8] U [100, 108]					-> 5
 [-9, -1] U [-101, -100] U [-113, -103]			-> -5
 [9, 14] U [120, 123] U [125, 129]			-> 8
 [-13, -10] U [-122, -120] U [-127, -124] U {-129}	-> -8
 {15}							-> 9
 [-32, -30]						-> -A
 [-40, -35] U [-51, -42] U [-58, -53] U
		[-83, -80] U {-64, -130, -140, -150}	-> -B

Other progress code are ignored. One symbol codes are displayed steady while
two-symbol codes are displayed using blinking. Boot progress codes are
displayed with decimal got unset (as opposed to 'display' command output).

Signed-off-by: Ilya Yanok <yanok@emcraft.com>