Update u-boot_git.bb, Correct U-Boot machine names

The latest revision of U-Boot requires changing the U-Boot machine names for
olimex based boards, as these were changed in U-Boot.

U-Boot tested on Olimex A20 Olinuxino

Signed-off-by: Jens Lucius <info@jenslucius.com>
Removed double KERNEL_IMAGETYPE

KERNEL_IMAGETYPE is defined in kernel (linux_3.4.bb) and machine
(conf/machine/include/sunxi.inc). This can lead to problems when
trying to change the image type. Removed from kernel.

Signed-off-by: Jens Lucius <info@jenslucius.com>
Merge pull request #68 from Noxenious/master

Update u-boot_git.bb, Correct U-Boot machine names