Add define for MAKE variable

Currently there is no define for MAKE. Running script always throws `obsolete make version; need GNU make 4.x or later` error.

This config is from [`build-util.sh` script](9d055fdac6/scripts/build-util.sh (L17-L19))

scripts/build-toolchains.sh

@@ -11,6 +11,10 @@ set -o pipefail
 DIR="$(dirname "$(readlink -f "${BASH_SOURCE[0]:-${(%):-%x}}")")"
 CHIPYARD_DIR="$(dirname "$DIR")"
 
+# Allow user to override MAKE
+[ -n "${MAKE:+x}" ] || MAKE=$(command -v gnumake || command -v gmake || command -v make)
+readonly MAKE
+
 usage() {
     echo "usage: ${0} [OPTIONS] [riscv-tools | esp-tools | ec2fast]"
     echo ""