First attempt; hand translated from Numato UCF file; currently untested.

Copied from targets/mimasv2, with random guesses as to what should
be removed from mimasv2 code.

In particular:
- Mimas v2 supports 100 MHz + 12 MHz clock; Elbert v2 has only 12 MHz clock
- Mimas v2 has external SDRAM; Elbert v2 has no external SDRAM
- Mimas v2 has physical programming/communications switch over shared USB link
- Elbert V2 has only programing over built in USB link; needs separate USBUART
  module to provide communications

**COMPLETELY UNTESTED** at this stage, have not even tried parsing Python...

On systems without external SDRAM (eg, Numato Elbert v2), there
is no MAIN_RAM_BASE, and we cannot hold a frame buffer as we
have only small amounts of in-FPGA SRAM.

This should be cleaned up to avoid even compiling the framebuffer /
pattern when there is no main ram / MAIN_RAM_BASE, but for now we
have stubbed out everything referring to main ram or L2 cache (since
the L2 cache is over the external SDRAM).

In theory this should still work on systems with MAIN RAM, but currently
untested.

UGLY!  Hand hacked linker.ld to allow linking on platform without
main_ram.

TODO: figure out way to generate this conditional on whether there
is main ram or not.

On platforms with main ram, we tell linker to assume that most things
will be in main_ram by the time we execute the code (and the boot
loader copies into the main ram).  On a platform without main ram,
we have to execute directly from the SPI flash (which is mapped
in a different location), and need to:
(a) ensure that the .text (code) and .rodata (const data) is in
the spi flash; and

(b) .data (updatable data) and .bss (stack) are in sram (static ram,
on FPGA cells)

Commented out cas submodule, as it wasn't being found for elbertv2
(perhaps due to changes needed in third_party/litex?); commented out
asserts on frequency copied from mimasv2 as it is unclear what they
should be on 12MHz clock (mimasv2 ones are based around 100Mhz clock)

At this point, with hand hacked linker.ld, we can compile and link
firmware ("make firmware"); but firmware is completely untested.