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
1 file changed