[fx3] Investigate/fix field checksums

[jynik]

While investigating a different issue, I found that the checksum test has been "turned off" in flash.c's NuandExtractField(). It smells like someone disabled it while debugging and never got back to it.

I'm registering this issue to follow up on this, and fix any checksum issues, if present.

[bglod]

Appears to still be the case:

bladeRF/fx3_firmware/src/flash.c

Lines 265 to 268 in a656abc

	a1 = *(unsigned short *)(&ub[c+1]); // read checksum
	a2 = crc16mp(0, ub, c+1); // calculated checksum
	if (a1 == a2 || 1) {

[rtucker]

Removing the || 1 caused a flashed FPGA to not load on my A4. This will take some additional thought. (Are we even writing the checksum?)