am335x: enable falcon boot mode for mmc (raw and fat) and nand

Jump into full u-boot mode if a 'c' character is received on the uart.

We need to adjust the spl bss/malloc area to not overlap with the
loadaddr of the kernel (sdram + 32k), so move it past u-boot instead.

For raw mmc, we store the kernel parameter area in the free space after
the MBR (if used). For nand, we use the last sector of the partition
reserved for u-boot.

This also enables the spl command in the full u-boot so the kernel
parameter area snapshot can be created.

Signed-off-by: Peter Korsgaard <peter.korsgaard@barco.com>