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/README.txt b/configs/README.txt
index 9e5a243..86496db 100644
--- a/configs/README.txt
+++ b/configs/README.txt
@@ -21,4 +21,4 @@
     make
 
 Note that the second method also works if you want to keep your custom
-configuration file outside the mbed TLS tree.
+configuration file outside the Mbed TLS tree.
diff --git a/configs/config-no-entropy.h b/configs/config-no-entropy.h
index 31fab4e..1964e8e 100644
--- a/configs/config-no-entropy.h
+++ b/configs/config-no-entropy.h
@@ -33,7 +33,7 @@
 #define MBEDTLS_HAVE_ASM
 #define MBEDTLS_HAVE_TIME
 
-/* mbed TLS feature support */
+/* Mbed TLS feature support */
 #define MBEDTLS_CIPHER_MODE_CBC
 #define MBEDTLS_CIPHER_PADDING_PKCS7
 #define MBEDTLS_ECP_DP_SECP256R1_ENABLED
@@ -47,7 +47,7 @@
 #define MBEDTLS_SELF_TEST
 #define MBEDTLS_VERSION_FEATURES
 
-/* mbed TLS modules */
+/* Mbed TLS modules */
 #define MBEDTLS_AES_C
 #define MBEDTLS_ASN1_PARSE_C
 #define MBEDTLS_ASN1_WRITE_C
diff --git a/configs/config-suite-b.h b/configs/config-suite-b.h
index 89898b3..56a700f 100644
--- a/configs/config-suite-b.h
+++ b/configs/config-suite-b.h
@@ -37,13 +37,13 @@
 #define MBEDTLS_HAVE_ASM
 #define MBEDTLS_HAVE_TIME
 
-/* mbed TLS feature support */
+/* Mbed TLS feature support */
 #define MBEDTLS_ECP_DP_SECP256R1_ENABLED
 #define MBEDTLS_ECP_DP_SECP384R1_ENABLED
 #define MBEDTLS_KEY_EXCHANGE_ECDHE_ECDSA_ENABLED
 #define MBEDTLS_SSL_PROTO_TLS1_2
 
-/* mbed TLS modules */
+/* Mbed TLS modules */
 #define MBEDTLS_AES_C
 #define MBEDTLS_ASN1_PARSE_C
 #define MBEDTLS_ASN1_WRITE_C
diff --git a/configs/config-thread.h b/configs/config-thread.h
index 0652136..e05b557 100644
--- a/configs/config-thread.h
+++ b/configs/config-thread.h
@@ -35,7 +35,7 @@
 /* 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
@@ -46,7 +46,7 @@
 #define MBEDTLS_SSL_DTLS_ANTI_REPLAY
 #define MBEDTLS_SSL_DTLS_HELLO_VERIFY
 
-/* mbed TLS modules */
+/* Mbed TLS modules */
 #define MBEDTLS_AES_C
 #define MBEDTLS_ASN1_PARSE_C
 #define MBEDTLS_ASN1_WRITE_C