[COMMANDS] Add a blockdev read command.

This patch adds a read command for block devices, allowing to read data from
it.
Arguments:
- the device name (mandatory)
- the amount of data in bytes to read. If not specified, it will take the
  device default block size.
- the offset in bytes.
[LIBS] Add a simple image loader for framebuffer splash screen.

This patch adds a simple image loader, managing only the P6 ppm format with
less that 256 colors, allowing framebuffer splash screen.
[COMMANDS] Adding the "fb image" command, to display an image on a framebuffer.

This "fb image" command takes the following argument:
- the framebuffer to display the image on
- the image path
- the "x", the horizontal offset on the screen
- the "y", the vertical offset on the screen