Add 16bpp BMP support

This patch adds 16bpp BMP support to the common lcd code.

Use CONFIG_BMP_16BPP and set LCD_BPP to LCD_COLOR16 to enable the code.

At the moment it's only been tested on the MIMC200 AVR32 board, but extending
this to other platforms should be a simple task !!

Signed-off-by: Mark Jackson <mpfj@mimc.co.uk>
Signed-off-by: Guennadi Liakhovetski <lg@denx.de>
Acked-by: Anatolij Gustschin <agust@denx.de>