board/beaglebone: fix kernel load command in u-boot environment file

the loaduimage command was renamed to loadimage when the default kernel
image changed from uImage to zImage in u-boot commit e3cf9692053a, which
made it into the v2013.10 u-boot release that the beaglebone_defconfig
currently builds.

Signed-off-by: Vanya Sergeev <vsergeev@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>