Build: Sprinkle some portability, fix on OpenBSD

realpath(1) is specific to coreutils and its behavior can be had
with readlink -f

Create the Toolchain Build directory if it doesn't exist before
calling readlink, since realpath(3) on at least OpenBSD will error
on a non-existent path

Meta/build-image-qemu.sh

@@ -19,7 +19,8 @@ if [ "$(uname -s)" = "Darwin" ]; then
 fi
 
 disk_usage() {
-    du -sm "$1" | cut -f1
+    # shellcheck disable=SC2003
+    expr "$(du -sk "$1" | cut -f1)" / 1024
 }
 
 DISK_SIZE=$(($(disk_usage "$SERENITY_ROOT/Base") + $(disk_usage Root) + 100))
@@ -33,7 +34,7 @@ printf "creating new filesystem... "
 if [ "$(uname -s)" = "OpenBSD" ]; then
     VND=$(vnconfig _disk_image)
     (echo "e 0"; echo 83; echo n; echo 0; echo "*"; echo "quit") | fdisk -e "$VND"
-    mkfs.ext2 -I 128 -F "/dev/${VND}i" || die "could not create filesystem"
+    newfs_ext2fs -D 128 "/dev/r${VND}i" || die "could not create filesystem"
 elif [ "$(uname -s)" = "FreeBSD" ]; then
     MD=$(mdconfig _disk_image)
     mke2fs -q -I 128 _disk_image || die "could not create filesystem"

Meta/build-root-filesystem.sh

@@ -85,7 +85,7 @@ mkdir -p mnt/dev
 echo "done"
 
 printf "writing version file... "
-GIT_HASH=$( (git log --pretty=format:'%h' -n 1 | head -c 7) || true )
+GIT_HASH=$( (git log --pretty=format:'%h' -n 1 | cut -c1-7) || true )
 printf "[Version]\nMajor=1\nMinor=0\nGit=%s\n" "$GIT_HASH" > mnt/res/version.ini
 echo "done"
 

Meta/write-only-on-difference.sh

@@ -22,7 +22,7 @@ trap cleanup 0 1 2 3 6
 "$@" > "${DST_FILE}.tmp"
 # If we get here, the command was successful, and we can overwrite the destination.
 
-if ! cmp --quiet -- "${DST_FILE}.tmp" "${DST_FILE}"; then
+if ! cmp -s -- "${DST_FILE}.tmp" "${DST_FILE}"; then
     # File changed, need to overwrite:
     mv -f -- "${DST_FILE}.tmp" "${DST_FILE}"
 fi