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/configs/config-thread.h b/configs/config-thread.h
index de24158..0de7e16 100644
--- a/configs/config-thread.h
+++ b/configs/config-thread.h
@@ -38,7 +38,7 @@
 /* System support */
 #define MBEDTLS_HAVE_ASM
 
-/* mbed TLS feature support */
+/* Mbed TLS feature support */
 #define MBEDTLS_AES_ROM_TABLES
 #define MBEDTLS_ECP_DP_SECP256R1_ENABLED
 #define MBEDTLS_ECP_NIST_OPTIM
@@ -50,7 +50,7 @@
 #define MBEDTLS_SSL_DTLS_HELLO_VERIFY
 #define MBEDTLS_SSL_EXPORT_KEYS
 
-/* mbed TLS modules */
+/* Mbed TLS modules */
 #define MBEDTLS_AES_C
 #define MBEDTLS_ASN1_PARSE_C
 #define MBEDTLS_ASN1_WRITE_C