arm64: zynqmp: accept an absolute path for PMUFW_INIT_FILE

The value of PMUFW_INIT_FILE is prefixed with "$(srctree)/", thus
forcing it to be a relative path inside the U-Boot source tree. Since
the PMUFW is a binary file generated outside of U-Boot, the PMUFW
binary must be copied inside the U-Boot source tree before the
build.

This generates a few problems:

 * if the source tree is shared among different out-of-tree builds,
   they will pollute (and potentially corrupt) each other
 * the source tree cannot be read-only
 * any buildsystem must add a command to copy the PMUFW binary
 * putting an externally-generated binary in the source tree is ugly
   as hell

Avoid these problems by accepting an absolute path for
PMUFW_INIT_FILE. This would be as simple as removing the "$(srctree)/"
prefix, but in order to keep backward compatibility we rather use the
shell and readlink to get the absolute path even when starting from a
relative path.

Since 'readlink -f' produces an empty string if the file does not
exist, we also add a check to ensure the file configured in
PMUFW_INIT_FILE exists. Otherwise the build would exit successfully,
but produce a boot.bin without PMUFW as if PMUFW_INIT_FILE were empty.

Tested in the 12 possible combinations of:
 - PMUFW_INIT_FILE empty, relative, absolute, non-existing
 - building in-tree, in subdir, in other directory

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Emmanuel Vadot <manu@bidouilliste.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>