add bootloaders build scripts

This adds the necessary tools to build bootloaders components (A-TF, U-Boot, OP-TEE).

Part of this was copied from [1] based on sha-1: 42636070ba0beb0701b3365b9fa1c3261a9623bf

[1] https://gitlab.com/mediatek/aiot/bsp/build-bootloaders

The README.md provide instructions on how to use scripts.

Signed-off-by: Julien Masson jmasson@baylibre.com Signed-off-by: souajih souajih@baylibre.com