ihex: add ihex2fw tool for converting HEX files into firmware images

Not the straight conversion to binary which objcopy can do for us, but
actually representing each record with its original {addr, length},
because some drivers need that information preserved.

Fix up 'firmware_install' to be able to build $(hostprogs-y) too.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>