pygrub: improve grub 2 support

pygrub: improve grub 2 support

* The "default" value can be a quoted string (containing an integer)
  so strip the quotes before interpreting.
* The "set" command takes a variable with an arbitrary name so instead
  of whitelisting the ones to ignore simply silently accept any set
  command with an unknown variable.
* Ignore the echo command.
* Handle the function { ... } syntax. Previously pygrub would error
  out with a syntax error on the closing "}" because it thought it was
  the closing bracket of a menuentry.

This makes pygrub2 work with the configuration files generated by
Debian Squeeze today.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>