binary_iso: Execute mkdir binary/.disk before writing to it

Before commit 9f3e5fe8d (Fix the way the .disk/mkisofs file is created)
all these commands (`mkdir`, write to `binary/.disk/mkisofs` and
`xorriso`) were in the same `binary.sh` script. Since that commit, the
write was extracted, to prevent issues with quoting, but the related
mkdir was left in `binary.sh`. This means that the write is now executed
first, and the `mkdir` only afterwards, making the `mkdir` quite pointless.

In practice, this did not break becaue binary_disk also does the same
`mkdir` and runs before `binary_iso`, but if one runs commands manually
and skips `binary_iso`, then this does break.

Even though this is not really a supported usecase, just move the mkdir
outside of `binary.sh`, so it runs *before* the write again as intended.

scripts/build/binary_iso

@@ -167,11 +167,11 @@ esac
 
 cat > binary.sh << EOF
 #!/bin/sh
-mkdir -p binary/.disk
 xorriso -as mkisofs ${XORRISO_OPTIONS} -o ${IMAGE} binary
 EOF
 
 # retain the xorriso command used to create the image in the image itself
+mkdir -p binary/.disk
 cat <<-EOF >binary/.disk/mkisofs
 xorriso -as mkisofs ${XORRISO_OPTIONS} -o ${IMAGE} binary
 EOF