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>
diff --git a/arch/arm/mach-k3/config.mk b/arch/arm/mach-k3/config.mk
index 7fc0b3f..be00d79 100644
--- a/arch/arm/mach-k3/config.mk
+++ b/arch/arm/mach-k3/config.mk
@@ -37,7 +37,7 @@
 ifeq ($(CONFIG_SYS_K3_KEY), "")
 KEY=u-boot-spl-eckey.pem
 else
-KEY=$(patsubst "%",%,$(CONFIG_SYS_K3_KEY))
+KEY=$(patsubst "%",$(srctree)/%,$(CONFIG_SYS_K3_KEY))
 endif
 
 u-boot-spl-eckey.pem: FORCE