keystone2: config: align names of images with MCSDK

The Multicore Software Development Kit (MCSDK) provides foundational
software for TI KeyStone II device platforms. It's supposed to be used
with uboot, and it's convenient to have the same names for images, so
correct environment image names according to the last MCSDK3.

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>
Reviewed-by: Tom Rini <trini@ti.com>