tools: mkimage: Check if file is regular file

Current Makefile.spl passes -R parameter which is not empty
and pointing to ./ folder.
"./tools/mkimage -T zynqmpimage -R ./"" -d spl/u-boot-spl.bin
spl/boot.bin"
That's why mkimage is trying to parse ./ file and generate
register init which is wrong.
Check that passed filename is regular file. If not do not work with it.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>