Add option to disable documentation

Documentation is typically auto-detected by checking for php and
doxygen. Add an option to explicitly disable it.

Remove toggle keywords from libraries, examples, documentation and
unit tests. They were not consistent with the default status.

Change-Id: I21049675ccfd8e58ac612cd058641b197db5c0eb

configure

@@ -20,9 +20,10 @@ show_help(){
     show_help_pre
     cat << EOF
 Advanced options:
-  ${toggle_libs}                  don't build libraries
-  ${toggle_examples}              don't build examples
-  ${toggle_unit_tests}            build unit tests
+  ${toggle_libs}                  libraries
+  ${toggle_examples}              examples
+  ${toggle_docs}                  documentation
+  ${toggle_unit_tests}            unit tests
   --libc=PATH                     path to alternate libc
   --as={yasm|nasm|auto}           use specified assembler [auto, yasm preferred]
   ${toggle_fast_unaligned}        don't use unaligned accesses, even when
@@ -285,6 +286,7 @@ CMDLINE_SELECT="
 
     libs
     examples
+    docs
     libc
     as
     fast_unaligned