builder: Allow to override some bsp settings in target

[andrzej-kaczmarek]

Target can now override some settings defined in BSP:

• bsp.compiler
• bsp.image_offset
• bsp.image_pad
• bsp.flash_map
• bsp.linkerscript
• bsp.part2linkerscript

To override, simply define the same key in target.yml and it will take precedence over the one defined in bsp.yml.