HBBL could be assembled using linker script rather than cat and dd #156

The Hostboot Boot Loader image is made up of a few things:

1. The Host Boot Boot Loader itself
2. The securerom image (the size of the image and then the image itself)
3. the hash of the hardware keys in the last 64 bytes of the image

This is currently constructed first using the linker script src/bootloader.ld for step 1, and subsequently manipulated in genPnorImages.pl to do 2 and 3.

There's no reason 2 and 3 couldn't be done by the linker script, which would give us the benefit of being able to catch HBBL and/or securerom growing beyond the maximum allowed HBBL size (which is restricted due to the size of the SBE SEEPROM)