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

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 <[email protected]>

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 <[email protected]>
Acked-by: Simon Glass <[email protected]>