xen/arm: Add support for booting gzip compressed uImages

xen/arm: Add support for booting gzip compressed uImages

At the moment, Xen does not support booting gzip compressed uImages.
This is because we are trying to decompress the kernel before probing
the u-boot header. This leads to a failure as the header always appears
at the top of the image (and therefore obscuring the gzip header).

Move the call to kernel_uimage_probe before kernel_decompress and make
the function self-containing by taking the following actions:
 - take a pointer to struct bootmodule as a parameter,
 - check the comp field of a u-boot header to determine compression type,
 - in case of compressed image, call kernel_decompress passing uImage
   header size as an offset to gzip header,
 - set up zimage.{kernel_addr,len} accordingly,
 - return -ENOENT in case of a u-boot header not found to distinguish it
   amongst other return values and make it the only case for falling
   through to try to probe other image types.

Modify kernel_decompress to take an additional parameter being an offset
to a gzip header from start address. This is needed so that a function
can first operate on a region containing actually compressed kernel (in case
of compressed uImage, size of u-boot header is an offset to a gzip header)
and then at the end pass the entire region (as it was before taking an offset
into account) to fw_unreserved_regions for freeing.

This approach avoids splitting the uImage probing into 2 stages (executed
before and after decompression) which otherwise would be necessary to
properly parse header, update boot module start and size before
decompression and update zimage.{kernel_addr,len} afterwards.

Remove the limitation from the booting.txt documentation.

Signed-off-by: Michal Orzel <michal.orzel@amd.com>
Reviewwed-by: Julien Grall <jgrall@amazon.com>