arm: K3: Fix usage of CONFIG_SYS_K3_KEY

For signing the tiboot3.bin image, an optional KEY file can be passed
using CONFIG_SYS_K3_KEY. Right now, Makefile scripts directly takes
the config value and uses it for signing. This is okay if the build
directory is a sub-directory of source tree, otherwise it fails.
Fix it by using the path relative to the source tree.

Reported-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>