Align uf2 blocks to flash erase pages

[timokroeger]

Fixes #762

The rp2040 bootrom uses the uf2 block number to determine when to erase a flash sector:
https://github.com/raspberrypi/pico-bootrom/blob/c09c7f08550e8a36fc38dc74f8873b9576de99eb/bootrom/virtual_disk.c#L205

When there is a gap in target addresses between two uf2 blocks this logic does not work out.
As a workaround align all uf2 blocks to page erase boundary, so that the bootrom always removes the correct sector.

[kilograham]

Argh; good catch on the bootrom bug.

Two things:

1. The changes should be limited to flash UF2 generation
2. I'm not sure the fix is quite right, because the block_num needs to change whenever the flash sector number does, and this is relative to the start of flash not the previous UF2 block address. I think a simpler, and the easier one conceptually, would be just to include zeroed data for any unused pages in a sector (they are going to be erased anyway).

[timokroeger]

Implemented your feedback, the code is much more readable now.

[kilograham]

Actually see #800