In barebox_non_pbl_start() we do not run at the address we are linked
at, so we must read linker variables using ld_var(). Since ld_var()
current is not available on arm64 we create two zero sized arrays,
one at the begin of the image and one at the end. The difference
between both is the image size we are looking for.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>