lk: Support for the Huawei U8800 board.

Parses boot.img/recovery.img from VFAT, if VFAT fails then tries to use
dedicated boot.img/recovery.img partitions.
aboot: Do not set serialno for U8800.

This is currently done on the kernel start.
u8800: Add support for offmode charging.
aboot: Show descriptive messages about booting mode.
u8800: Initialize display before keys.
u8800: Show descriptive message before waiting for modem-up.
u8800: Wait before scanning keys.

A small delay is added to give the use more time to press the keys.
This delay does not increase booting time as the bootloader waits for modem-up
after processing keys.
u8800: Print out booting device.