apollo_fpga: Flash programming speed improvements

[mndza]

Improves flashing speed by:

• Ignoring unused SPI responses.
• Avoiding an unnecessary check.
• Performing partial erases when programming.

9 implements partial erases too, but the version in this PR is also able to use mixed eraseblock sizes.

Offset writes and reads are intentionally left out of this PR, but it's open for discussion.

[mndza]

Added ability to use offsets in flash reads/writes