x509: move common functions to x509 helper

Move x509_check_for_self_signed as a common helper function
that can be shared by legacy crypto lib and MbedTLS implementation.

Signed-off-by: Raymond Mao <raymond.mao@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/lib/crypto/Makefile b/lib/crypto/Makefile
index 4ad1849..946cc3a 100644
--- a/lib/crypto/Makefile
+++ b/lib/crypto/Makefile
@@ -37,6 +37,7 @@
 	x509.asn1.o \
 	x509_akid.asn1.o \
 	x509_cert_parser.o \
+	x509_helper.o \
 	x509_public_key.o
 
 $(obj)/x509_cert_parser.o: \