tests/pkg_mbedtls-blacklist: blacklist samr21-xpro

The ENTROPY test always fails on this board

main(): This is RIOT! (Version: buildtest)
mbedtls test

  SHA-224 test #1: passed
  SHA-224 test #2: passed
  SHA-224 test #3: passed
  SHA-256 test #1: passed
  SHA-256 test #2: passed
  SHA-256 test #3: passed

  ENTROPY test: failed

tests/pkg_mbedtls/Makefile

@@ -3,6 +3,9 @@ include ../Makefile.tests_common
 USEPKG += mbedtls
 USEMODULE += mbedtls_entropy
 
+# entropy test keeps failing on this board due to low entropy
+TEST_ON_CI_BLACKLIST += samr21-xpro
+
 # Increase stack size of main thread.
 CFLAGS += -DTHREAD_STACKSIZE_MAIN=THREAD_STACKSIZE_LARGE