hw/arm/boot: take Linux/arm64 TEXT_OFFSET header field into account

hw/arm/boot: take Linux/arm64 TEXT_OFFSET header field into account

The arm64 boot protocol stipulates that the kernel must be loaded
TEXT_OFFSET bytes beyond a 2 MB aligned base address, where TEXT_OFFSET
could be any 4 KB multiple between 0 and 2 MB, and whose value can be
found in the header of the Image file.

So after attempts to load the arm64 kernel image as an ELF file or as a
U-Boot image have failed (both of which have their own way of specifying
the load offset), try to determine the TEXT_OFFSET from the image after
loading it but before mapping it as a ROM mapping into the guest address
space.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1489414630-21609-1-git-send-email-ard.biesheuvel@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>