Add more details to the BROM's boot function

The boot function loops over the various possible boot media (MMC0,
NAND, SPINOR, MMC3) and tries to load the SPL. This patch adds comments
about each step of the 'boot' function. Also remove for example.

Signed-off-by: Oliver Schinagl <oliver@schinagl.nl>
BROM: add pseudo C code for start/boot

While boot (shouldn't it be start really?) still contains some asm by
necessity this should be relative accurate C code.

Signed-off-by: Oliver Schinagl <oliver@schinagl.nl>
BROM: add pseudo code for boot()

This patch adds C-pseudo code from boot()

Signed-off-by: Oliver Schinagl <oliver@schinagl.nl>
BROM: if check_uboot() is true we actually go to FEL mode, not on
failure of try_mmc

Signed-off-by: Oliver Schinagl <oliver@schinagl.nl>