Reported by Nikolaus Schaller, Jan 26, 2010

The target is to boot as fast as possible in default mode but allow 
the user to easily break into a boot menu.

The GTA02 solves this by pressing the AUX button while switching on.

This can be kept. The only things we have to do
a) detect AUX button before the 10 seconds loop is started where 
u-boot waits for console activity
b) if no AUX -> boot immediately to the default boot command
c) if AUX is pressed, we must alternatively wait for a TSC press or 
console activity (so the 10 seconds counter will be removed)

We must be able to load a 'preboot' command that initializes the LCM 
and loads a splash screen/boot menu screen from the boot medium.

Note: on the GTA04 (but not GTA04E) AUX is also connected to the 
OMAP3530 boot options. Therefore, the boot order will be reversed 
from

1. NAND  
2. USB  
3. UART3  
4. MMC1 

to

1. USB 
2. UART 
3. MMC1
4. NAND

This is good for recovering from a damaged u-boot in NAND. Just 
insert a working u-boot and press AUX while booting.