MEGAFLASH: refactor low level flashing interface

[lydon42]

In this issue we combine all the low level changes that are still needed to make MEGAFLASH work again with non-r3a+ boards.

This means: support different QSPI flash chips, support flashing and CRC32 calculation without Attic RAM, hardware acceleration, and other convenient feature that could not been added to Release 0.96 with #683

Generic goals:

• [x] fix mega65r3 (non-A) flashing (different QSPI)
• [x] fix mega65r2 flashing (should work, untested)
• [x] fix all the boards (nexys, wukong) flashing (different QSPI, no Attic RAM)
• [x] use hwaccel for integrated flasher
• [x] make atticram switchable for standalone version
  • [ ] crc32 feedback is missing
• [ ] make bitbash switchable for standalone version
• [ ] visual feedback for verify step (progress and border)
  • [ ] make border flash optional? I like flashing borders...

Sub-Issues:

• [ ] #592
• [x] #522
• [x] #762
• [x] #763
• [x] #764

Optional:

• [ ] a better way to change flags (i.e. not having to change flags on multiple slots)
• [ ] #711
• [ ] #750

[lydon42]

Next Steps:

• remove attic RAM requirement for whole core caching, perhaps using $50000 as a 64k sliding cache instead (lydon)
• enable crc32 calc without having to use the 512byte buffers already used by flashing and sdcard reading, perhaps by making it use $40000 and 32bit offset pointers (lydon)
• abstract lowlevel qspi interface and making most of the rest of qspicommon.c obsolete (0xa000)