[J-core] ROM image generator.

[Rob Landley]

We have ram.sh which calls make -C testrom, calls sh2-elf-size for no reason,
uses sh2-elf-objcopy on a binary it magically knows the name of out of the
subdir (will generic objcopy not work?) to de-ELF it, and then builds and calls
conf.c to turn that into a vhdl initializer for an sram block so it can pretend
to be a boot ROM for ice40.

I was thinking I should modify conv.c to have 8 constants per line instead of
one so when we check in the generated file (so as not to require people to have
sh2-elf installed in order to build the bitstream), it doesn't DOMINATE THE DIFF
quite so much. (Pages and pages and pages of change...)

But the other thing I noticed is we have a hardwired ROM size in conv.c and
another hardwired ROM size in cpu_simple_sram.vhd, and they have to match. The
"type rom_t is array (0 to 16383)" has to match "generic (ADDR_WIDTH : natural
:= 15);".

(They have to match in a slightly non-obvious way: rom_t is array of 32 bit
values, ADDR_WIDTH is number of 16 bit instructions, so 1<<15 is 32768 16 bit
_instructions_ stored in 16384 32-bit words.)

Anywhat, I want to have rom.sh objcopy the binary, figure out how big the result
is (with power of 2 round up, of course), write out a ROM that big, and then
have cpu_simple_sram.vhd use that value, so whatever binary you feed rom.sh
(which should be a command line argument) just works. (At least in simulation,
the lattice tools will still run out of RAM if it's too big, only the little
ones are initializable.)

Except then cpu_simple_sram needs to import something to get ADDR_WIDTH, and
just "use work.ram_init;" seems dubious? (Can I do that? It's got a package in
it, not just definitions: use work.ram_init.ADDR_WIDTH maybe?)

Also, we should have the testrom dir's Makfilie do the objcopy. Also, we should
check in the resulting -o binary file as the generated thing we commit to the
repo instead of the VHDL, and have lattice_whatsis.sh call conv.c and recreate
the vhd file from the binary each build.

And we might want to break down and have a top level Makefile at some point, if
we've got it in the subdirectory...

Rob