[Pal,LibOS] New logging system

Instead of 'loader.debug_type', introduce 'loader.log_level'
and 'loader.log_file', along with a set of definitions for
logging at a chosen level.

For now, the call sites keep using the legacy macros (SGX_DBG and
debug()), because converting them all will conflict with other
big changes in the code base. The existing LibOS calls are
assumed to be at 'info' level.

Examples/python-simple/python.manifest.template

@@ -3,11 +3,13 @@
 # This manifest was prepared and tested on Ubuntu 16.04/18.04 and tested with
 # Python 3.5 and 3.6.
 
-# Graphene environment, including the path of the library OS and the debug
-# option (inline/none).
-loader.preload = "file:$(GRAPHENEDIR)/Runtime/libsysdb.so"
 libos.entrypoint = "file:$(PYTHONEXEC)"
-loader.debug_type = "$(GRAPHENEDEBUG)"
+
+# Path to the library OS
+loader.preload = "file:$(GRAPHENEDIR)/Runtime/libsysdb.so"
+
+# Graphene log level
+loader.log_level = "$(GRAPHENE_LOG_LEVEL)"
 
 # Read application arguments directly from the command line. Don't use this on production!
 loader.insecure__use_cmdline_argv = 1