openblocks_a6_defconfig: improve/fix configuration

openblocks_a6_defconfig: improve/fix configuration

The board has an ethernet interface, so enable DHCP for it.

The used flash chip uses 16K erase blocks, so use that for JFFS2:

mtdinfo -a
..
mtd5
Name:                           user
Type:                           nand
Eraseblock size:                16384 bytes, 16.0 KiB
Amount of eraseblocks:          1915 (31375360 bytes, 29.9 MiB)
Minimum input/output unit size: 512 bytes
Sub-page size:                  256 bytes
OOB size:                       16 bytes
Character device major/minor:   90:10
Bad blocks are allowed:         true
Device is writable:             true

While we're at it, add a readme explaining how to build and upload the build
to the board.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>