add file detection support ...

Several filetypes can be autodetected. Barebox could make use
of this in several ways:

- Add a command to detect filetypes
- detect arm zImages and uImages to unify the different boot commands
- maybe detect UBI or JFFS2 images to construct parts of the kernel
  command line
- select correct uncompression function based on filetype

This patch adds basic support to detect filetypes.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>