bootloaders: layouts and sequence: Improve the figures

I believe there are several issues within the figures, aside from the
color choices which can sometimes be misleading:
- The lack of OP-TEE box
- A global misuse of the arrows style
- No description of TIFS nor DM
- The mention of SYSFW which is nowhere otherwise
- The identification of TF-A (A53) as BL33
- The lack of bundle around the u-boot.img
- The TIFS markes as BL1 (it cannot be a BL1, maybe a BL31 at most), but
  this is very TI specific anyway).

Signed-off-by: Miquel Raynal <[email protected]>