give make a single call for the settings the CLI resolves

myos export prints every setting the requested stacks declare, in one process:
68 variables in half a second, where asking for them one at a time costs a
process each and about ten seconds.

The shim reads that once and includes it, so a .mk target sees
HOST_FABIO_VERSION and the computed fabio tag without make knowing anything
about how they are produced. --make quotes the values so make neither expands
a $ nor starts a comment at a #.

lib/cmd/export.sh

@@ -0,0 +1,36 @@
+#shellcheck shell=sh
+# myos export  print every setting the requested stacks declare, as KEY=value.
+#
+# One call, so a Makefile can read the whole set at once:
+#   $(eval $(shell myos export STACK=host))
+# Asking for each variable separately costs a process per variable.
+myos_cmd_export() {
+  # shellcheck disable=SC2209  # these are literal format names, not commands
+  _fmt=sh
+  # shellcheck disable=SC2209
+  case ${MYOS_ARGS:-}${MYOS_VARS:-} in *--make*) _fmt=make ;; esac
+  for _v in $(myos_declared_defaults); do
+    _val=$(myos_var "$_v")
+    case $_fmt in
+      make)
+        # make would expand a $ and start a comment at a #, and := stops it
+        # from expanding the value again later
+        printf '%s := %s\n' "$_v" "$(printf '%s' "$_val" | sed -e 's/\$/$$/g' -e 's/#/\\#/g')" ;;
+      *) printf '%s=%s\n' "$_v" "$_val" ;;
+    esac
+  done
+}
+
+# myos_declared_defaults  the variables the loaded hooks declare
+myos_declared_defaults() {
+  for _ref in $MYOS_STACKS; do
+    for _d in $(myos_stack_dirs "$_ref"); do
+      for _f in "$_d"/_stack.sh "$_d/$(myos_stack_name "$_ref").sh"; do
+        [ -f "$_f" ] && sed -n 's/^myos_default_\([A-Za-z_][A-Za-z0-9_]*\)().*/\1/p' "$_f"
+      done
+      for _f in "$_d"/_stack.env "$_d/$(myos_stack_name "$_ref").env"; do
+        [ -f "$_f" ] && myos_dotenv_parse "$_f" | sed 's/=.*//'
+      done
+    done
+  done | sort -u
+}