TA dev kit: add support for TA encryption

Add CFG_ENCRYPT_TA as TA build time configuration option to enable
encryption of TA using encryption key provided via TA_ENC_KEY build
time option. The default value of TA_ENC_KEY is derived from 16 zero
bytes default hardware unique key.

Also rename scripts/sign.py to scripts/sign_encrypt.py to reflect
optional encryption support along with signing of TAs.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/mk/lib.mk b/mk/lib.mk
index 13ace45..c5d07a2 100644
--- a/mk/lib.mk
+++ b/mk/lib.mk
@@ -43,7 +43,7 @@
 libnames	:= $(libname) $(libnames)
 libdeps		:= $(lib-libfile) $(libdeps)
 
-SIGN = scripts/sign.py
+SIGN = scripts/sign_encrypt.py
 TA_SIGN_KEY ?= keys/default_ta.pem
 
 define process-lib