configs: ti_armv7_keystone2: switch to using kernel zImage

Switch to using zImage instead of uImage. and while at it, start using
bootz as default. While at it, get rid of BOOTIMAGE define and start
using Linux upstream dtb file names.

Reviewed-by: Murali Karicheri <m-karicheri2@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Nishanth Menon <nm@ti.com>