configs: ti_armv7_keystone2: replace addr_uboot with loadaddr

This patch replaces not existing addr_uboot environment variable by
loadaddr at get_uboot_net and burn_uboot_xxx commands. Otherwise these
commands are broken.

Signed-off-by: Vitaly Andrianov <vitalya@ti.com>
Acked-by: Nishanth Menon <nm@ti.com>