Meta: Fix shellcheck warnings in various scripts

Warnings fixed:
 * SC2086: Double quote to prevent globbing and word splitting.
 * SC2006: Use $(...) notation instead of legacy backticked `...`
 * SC2039: In POSIX sh, echo flags are undefined
 * SC2209: Use var=$(command) to assign output (or quote to assign string)
 * SC2164: Use 'cd ... || exit' or 'cd ... || return' in case cd fails
 * SC2166: Prefer [ p ] && [ q ] as [ p -a q ] is not well defined.
 * SC2034: i appears unused. Verify use (or export if used externally)
 * SC2046: Quote this to prevent word splitting.
 * SC2236: Use -z instead of ! -n.

There are still a lot of warnings in Kernel/run about:
 - SC2086: Double quote to prevent globbing and word splitting.

However, splitting on space is intentional in this case, and not trivial to
change. Therefore ignore the warning for now - but we should fix this in
the future.

Kernel/makeall.sh

@@ -9,7 +9,7 @@ while [ "$1" != "" ]; do
     case $1 in
         -f | --fast )           fast_mode=1
                                 ;;
-        -h | --help )           echo "-f or --fast: build fast without cleaning or running tests"
+        -h | --help )           printf -- "-f or --fast: build fast without cleaning or running tests\n"
                                 exit 0
                                 ;;
     esac
@@ -18,10 +18,10 @@ done
 
 sudo id
 
-MAKE=make
+MAKE="make"
 
 if [ "$(uname -s)" = "OpenBSD" ]; then
-	MAKE=gmake
+	MAKE="gmake"
 fi
 
 if [ "$fast_mode" = "1" ]; then