make_version: Fix check for '.version' file

When building from the non-git tree and with '.version' file missing,
make_version shows this bash error message.

% make
...
cat: .version: No such file or directory
...
error You need to set SKIBOOT_VERSION environment variable

This patch fixes this issue by updating '.version' file check in
make_version script to test if it exists as a regular file and is
of non-zero size.

Fixes: 6a3cad2("Add fallback to .version if not git and no SKIBOOT_VERSION")
Signed-off-by: Vaibhav Jain <[email protected]>
Signed-off-by: Stewart Smith <[email protected]>

make_version.sh

@@ -50,7 +50,7 @@ else
 	then
 		echo $SKIBOOT_VERSION
 	else
-		if [ ! -z "`cat .version`" ];
+		if [ -f ".version" -a -s ".version" ];
 		then
 			cat .version
 		else