Look for documentation only in specific directories

Generate the documentation from include and doxygen/input only. Don't
get snared by files containing Doxygen comments that lie in other
directories such as tests, yotta, crypto/include, ...

The only difference this makes in a fresh checkout is that the
documentation no longer lists target_config.h. This file is from
yotta, does not contain any Doxygen comment, and its inclusion in the
rendered documentation was clearly an oversight.

doxygen/mbedtls.doxyfile

@@ -664,7 +664,7 @@ WARN_LOGFILE           =
 # directories like "/usr/src/myproject". Separate the files or directories
 # with spaces.
 
-INPUT                  = ..
+INPUT                  = ../include input
 
 # This tag can be used to specify the character encoding of the source files
 # that doxygen parses. Internally doxygen uses the UTF-8 encoding, which is
@@ -696,7 +696,7 @@ RECURSIVE              = YES
 # Note that relative paths are relative to the directory from which doxygen is
 # run.
 
-EXCLUDE                = ../configs ../yotta/module
+EXCLUDE                =
 
 # The EXCLUDE_SYMLINKS tag can be used to select whether or not files or
 # directories that are symbolic links (a Unix file system feature) are excluded