What does "NAND read from offset a0000 failed -117" mean in U-Boot? The board still boots correctly.

Error code -117 means there were bit flips in the read memory region, but the ECC algorithm could correct them. So the image is read correctly. A NAND page (typically 512 bytes) will become unreadable, if more bits flip than the ECC algorithm can handle, so depending on the situation it may make sense to rewrite the corresponding block. The standard images handle 1 bit error per NAND page, but a firmware handling 4 bit errors (or with small adjustments even more) is also available: http://www.armbedded.eu/node/675.