Implement charging mode

This is the initial implementation of the charging mode.

• The charging-sdl is packaged using the latest commit, a tagged version would be better.
• Added charging-sdl and triggerhappy to the initramfs-extra
• init script changes:
  • Extracted common code to configure directfb and tslib in a function
  • Check charging boot mode against a list of cmdline options
  • Use the same font file configured for osk-sdl
  • Configure triggerhappy to start charging-sdl (if it's not already running) when the power key is pressed
  • Start charging-sdl once before starting triggerhappy (to show something instead of the loading splash screen)

Please review the code and test it on some devices.

Close: #1081 (closed)