Get SPI flash working on BaseSoC with the various platforms

[mithro]

Get the gateware working which allows the lm32 firmware to access the SPI flash.

• [x] Numato Opsis
• [x] Digilent Atlys
• [x] Pipistrello

See https://github.com/timvideos/HDMI2USB-misoc-firmware/issues/113 and #236

[mithro]

Useful links;

• https://github.com/m-labs/misoc/blob/legacy/misoclib/mem/flash/spiflash.py
• https://github.com/m-labs/artiq/blob/master/artiq/runtime/flash_storage.h
• https://github.com/m-labs/artiq/blob/master/artiq/runtime/flash_storage.c
• https://github.com/m-labs/artiq/blob/master/artiq/frontend/artiq_mkfs.py
• https://github.com/m-labs/migen/blob/master/migen/build/platforms/lx9_microboard.py#L34

[mithro]

Another thing to notice is that SD Cards can actually be operated in SPI Flash mode...

[mithro]

The Atlys, Opsis and Pipistrello all have working SPI flash for booting the firmware now.

[mithro]

SPI flash is now working on all our supported platforms.