openssl: add option to provide the openssl path

Add a new variable to have the directory that contains the openssl
libraries needed for xtest. The build system will look for the needed
libraries specified in this variable during the compilation of xtest.

Signed-off-by: Davidson Kumaresan <davidson.kumaresan@arm.com>

host/xtest/Makefile

@@ -40,7 +40,7 @@ endef
 WITH_OPENSSL ?= y
 ifeq ($(WITH_OPENSSL),y)
 CFLAGS += -DOPENSSL_FOUND=1 -DOPENSSL_API_COMPAT=10100
-LDFLAGS += -lcrypto
+LDFLAGS += -L$(OPENSSL_DIR)/lib -lcrypto
 endif #require OpenSSL
 
 srcs := regression_1000.c