configs/olimex_imx233_olinuxino: switch to u-boot

configs/olimex_imx233_olinuxino: switch to u-boot

mxs-bootlets broke somewhere in between toolchain version bumps, leading to
boot failure.

Directly boot u-boot, as this is supported now, but it requires updated
partitioning (see readme.txt)

Drop mxs-bootlets patches, they aren't needed anymore.

Kernel is built as an uImage because the u-boot defconfig expects an
uImage on the second partition.

Tested on iMX233-OLinuXino-MINI hardware.

[Peter: adjust readme/config as suggested by Arnout]
Signed-off-by: Phil Eichinger <phil@zankapfel.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>