Add a test of MBEDTLS_CONFIG_FILE

configs/README.txt documents that you can use an alternative
configuration file by defining the preprocessor symbol
MBEDTLS_CONFIG_FILE. Test this.
diff --git a/tests/scripts/all.sh b/tests/scripts/all.sh
index 22c8129..89e0c7e 100755
--- a/tests/scripts/all.sh
+++ b/tests/scripts/all.sh
@@ -1032,6 +1032,17 @@
     make SHARED=1 all check -j1
 }
 
+component_build_mbedtls_config_file () {
+    msg "build: make with MBEDTLS_CONFIG_FILE" # ~40s
+    # Use the full config so as to catch a maximum of places where
+    # the check of MBEDTLS_CONFIG_FILE might be missing.
+    scripts/config.pl full
+    sed 's!"check_config.h"!"mbedtls/check_config.h"!' <"$CONFIG_H" >full_config.h
+    echo '#error "MBEDTLS_CONFIG_FILE is not working"' >"$CONFIG_H"
+    make CFLAGS="-I '$PWD' -DMBEDTLS_CONFIG_FILE='\"full_config.h\"'"
+    rm -f full_config.h
+}
+
 component_test_m32_o0 () {
     # Build once with -O0, to compile out the i386 specific inline assembly
     msg "build: i386, make, gcc -O0 (ASan build)" # ~ 30s