Update spelling "mbed TLS" to "Mbed TLS"

The official spelling of the trade mark changed from all-lowercase "mbed"
to normal proper noun capitalization "Mbed" a few years ago. We've been
using the new spelling in new text but still have the old spelling in a
lot of text. This commit updates most occurrences of "mbed TLS":

```
sed -i -e 's/mbed TLS/Mbed TLS/g' $(git ls-files ':!ChangeLog' ':!tests/data_files/**' ':!tests/suites/*.data' ':!programs/x509/*' ':!configs/tfm*')
```

Justification for the omissions:

* `ChangeLog`: historical text.
* `test/data_files/**`, `tests/suites/*.data`, `programs/x509/*`: many
  occurrences are significant names in certificates and such. Changing
  the spelling would invalidate many signatures and tests.
* `configs/tfm*`: this is an imported file. We'll follow the upstream
  updates.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/compat.sh b/tests/compat.sh
index 9529975..ba8465b 100755
--- a/tests/compat.sh
+++ b/tests/compat.sh
@@ -276,7 +276,7 @@
 {
     if [ "X" != "X$FILTER" -o "X" != "X$EXCLUDE" ];
     then
-        # Ciphersuite for mbed TLS
+        # Ciphersuite for Mbed TLS
         M_CIPHERS=$( filter "$M_CIPHERS" )
 
         # Ciphersuite for OpenSSL
@@ -286,7 +286,7 @@
         G_CIPHERS=$( filter "$G_CIPHERS" )
     fi
 
-    # For GnuTLS client -> mbed TLS server,
+    # For GnuTLS client -> Mbed TLS server,
     # we need to force IPv4 by connecting to 127.0.0.1 but then auth fails
     if is_dtls "$MODE" && [ "X$VERIFY" = "XYES" ]; then
         G_CIPHERS=""