fix: omit printing an error on mbedtls not cloned

Using Makefile's wildcard statement in place of a shell find command to
prevent the error message below when mbedtls submodule is not cloned:

    find: ‘ext/mbedtls/include’: No such file or directory

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: I9677aa4e97485b597e00ba0bdf76967f9093afe9
diff --git a/lib/ext_mbedtls/mbedtls.mk b/lib/ext_mbedtls/mbedtls.mk
index 1845bf9..74ab284 100644
--- a/lib/ext_mbedtls/mbedtls.mk
+++ b/lib/ext_mbedtls/mbedtls.mk
@@ -9,9 +9,9 @@
 $(error Error: MBEDTLS_DIR not set)
 endif
 
-MBEDTLS_CHECK := $(shell find ${MBEDTLS_DIR}/include -name mbedtls)
+MBEDTLS_PRESENT := $(wildcard ${MBEDTLS_DIR}/include/mbedtls)
 
-ifneq (${MBEDTLS_CHECK},)
+ifneq (${MBEDTLS_PRESENT},)
 $(info Found mbedTLS at ${MBEDTLS_DIR})
 
 TFTF_INCLUDES += -I${MBEDTLS_DIR}/include
@@ -51,5 +51,5 @@
 		x509_crt.c 			\
 		)
 else
-$(info MbedTLS not found, some dependent tests will be skipped or failed.)
-endif
\ No newline at end of file
+$(info MbedTLS not found, some dependent tests will be skipped or fail.)
+endif