Fix getting failure of softlink default_ta.pem

Currently, default secret key default.pem
and default_ta.pem is softlink to it.
But this link creation gets failed easily due to many reason.

Although we can set this config in make args during build,
But for simply, just change to refer to default.pem.

Signed-off-by: Dien Pham <dien.pham.ry@renesas.com>

mk/config.mk

@@ -219,7 +219,7 @@ _CFG_WITH_SECURE_STORAGE := $(call cfg-one-enabled,CFG_REE_FS CFG_RPMB_FS)
 # When performing external HSM signing for TA's TA_SIGN_KEY can be set to dummy
 # key and then set TA_PUBLIC_KEY to match public key from the HSM.
 # TA_PUBLIC_KEY's public key will be embedded into OP-TEE OS.
-TA_SIGN_KEY ?= keys/default_ta.pem
+TA_SIGN_KEY ?= keys/default.pem
 TA_PUBLIC_KEY ?= $(TA_SIGN_KEY)
 
 # Subkeys is a complement to the normal TA_SIGN_KEY where a subkey is used