Scripts to create custom SPL/UBOOT and FIT files

To use the scripts you can use the following make targets:
```
#if creating a custom fit image call this
make fit 
or
#to pass a different cross compiler use; it defaults to arm-linux-gnueabi-
make CROSS_COMPILE=arm-linux-gnueabihf- fit
or if you only need to create a custom uboot then call
make uboot
```
All the generated files get created on a new folder called `bintest`