The current way to calculate the compressed image position works
with the OSELAS toolchains, but not with the Debian/Ubuntu toolchains.
For these toolchains &image_end_marker already returns the correct
address, adding global_variable_offset() to it is wrong.

The solution seems to be to put the image_end_marker into a different
object file so that the compiler cannot play any tricks to resolve
the address internally in the object file.

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

The ld_var solves the issue that when compiled with -pie the linker
provided variables are all 0x0. This mechanism however refuses to
compile with aarch64 support.

This patch replaces the ld_var mechanism with a nice little trick
learned from U-Boot: Instead of using linker provided variables
directly with "__bss_start = ." we put a zero size array into
a separate section and use the address of that array instead of
the linker variable. This properly works before relocation.

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