Makefile: use $(shell ...) for determining file_size

file_size was being calculated using back-ticks but map_size uses
$(shell ...). Update the file_size calculation to use $(shell ...).

From: Jeroen Hofstee <jeroen@myspectrum.nl>

The binary_size_check target relies on stat -c %s
to return the size of u-boot.bin. This only works
with GNU stat though. Use wc instead.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>