bootmeth: Update the read_file() method to include a type

bootmeth: Update the read_file() method to include a type

We want to record the type of each file which is loaded. Add an new
parameter for this, to the read_file() method. Update all users.

Make bootmeth_common_read_file() store information about the image that
is read.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>