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/library/CMakeLists.txt b/library/CMakeLists.txt
index 147caaf..1bff6bc 100644
--- a/library/CMakeLists.txt
+++ b/library/CMakeLists.txt
@@ -1,7 +1,7 @@
-option(USE_STATIC_MBEDTLS_LIBRARY "Build mbed TLS static library." ON)
-option(USE_SHARED_MBEDTLS_LIBRARY "Build mbed TLS shared library." OFF)
-option(LINK_WITH_PTHREAD "Explicitly link mbed TLS library to pthread." OFF)
-option(LINK_WITH_TRUSTED_STORAGE "Explicitly link mbed TLS library to trusted_storage." OFF)
+option(USE_STATIC_MBEDTLS_LIBRARY "Build Mbed TLS static library." ON)
+option(USE_SHARED_MBEDTLS_LIBRARY "Build Mbed TLS shared library." OFF)
+option(LINK_WITH_PTHREAD "Explicitly link Mbed TLS library to pthread." OFF)
+option(LINK_WITH_TRUSTED_STORAGE "Explicitly link Mbed TLS library to trusted_storage." OFF)
 
 # Set the project root directory if it's not already defined, as may happen if
 # the library folder is included directly by a parent project, without