ihex: Add support for long records to ihex2fw.c

Some drivers could do with using records like Intel HEX, but with each
record being larger than 256 bytes. This has been possible in the binary
representation (struct ihex_binrec) in the kernel since the beginning --
at least of the the current version of history. But we haven't been able
to represent that in the .HEX files which get converted to .fw files.

This adds a '-w' option to ihex2fw to make it interpret the first _two_
bytes of each line as the record length, instead of only one byte. And
adds makefile rules for %.H16->%.fw which use that.

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