jffs2: some fixes to summary support

This patch fixes some issues with JFFS2 summary support in U-Boot.
1/ Summary support made compilation configurable (as summary support
considered expiremental even in Linux).
2/ Summary code can do unaligned 16-bit and 32-bit memory accesses.
We need to get data byte by byte to exclude data aborts.
3/ Make summary scan in two passes so we can safely fall back to full
scan if we found unsupported entry in the summary.

Signed-off-by: Ilya Yanok <yanok@emcraft.com>