arm: add command line option for the total image size

Currently, kexec on arm assumes that it's safe to place binary images
such as atags, dtb or initrd at an estimated offset after the load
address. That estimated offset is set to 4 times the size of the
compressed image, hence assuming a minimum compression ratio of 1:4.

While that assumption matches what the in-kernel compressors are able to
achive, it doesn't take into account the .bss section the kernel image
carries, and which can grow to arbitrary sizes while not accounting to
the compressed image size.

After decompression, and before the execution of the compressed kernel,
the .bss area is initialized to zeros, trampeling over the binary images
in case they happen to live in that piece of memory.

Unfortunately, determining the full image size is not easiliy possible
at runtime, as it would include doing all possible ways of
decompression and then walk the ELF sections by hand.

For now, allow users to override the static offset with a new, arm
specific command line argument. Users are supposed to set this, and
determine a sane value by using 'arm-linux-size vmlinux'.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Simon Horman <horms@verge.net.au>

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