ARM: verify size of zImage

The linker can sometimes add additional sections to the zImage ELF file
which results in the zImage binary being larger than expected.  This
causes appended DT blobs to fail.

Verify that the zImage binary is the expected size, and fail the build
if this is not the case.

We need to include the .data.rel.ro section in the image as the RiscPC
build includes font data that contains a small amount of data in this
section.

Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>