[build] Allow for construction of a text file containing the version

Add a rule to construct bin/version.txt containing the version number,
to allow a GitHub Actions workflow to verify that a tagged release
embeds a version number that matches the tag.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

.github/workflows/build.yml

@@ -42,6 +42,7 @@ jobs:
                ${{ env.bindir }}/ipxe-legacy.lkrn \
                ${{ env.bindir }}/ipxe-legacy.pxe \
                ${{ env.bindir }}/undionly.kpxe \
+               ${{ env.bindir }}/version.txt \
                ${{ env.bindir }}/errors
 
       - name: Hardware list
@@ -63,6 +64,7 @@ jobs:
             src/${{ env.bindir }}/ipxe-legacy.lkrn
             src/${{ env.bindir }}/ipxe-legacy.pxe
             src/${{ env.bindir }}/undionly.kpxe
+            src/${{ env.bindir }}/version.txt
             src/${{ env.bindir }}/errors
             src/${{ env.bindir }}/niclist.txt
 

src/Makefile.housekeeping

@@ -1094,6 +1094,11 @@ $(BIN)/NIC : $(AUTO_DEPS)
 	@perl -ne 'chomp; print "$$1\n" if /\# NIC\t(.*)$$/' $^ >> $@
 CLEANUP		+= $(BIN)/NIC	# Doesn't match the $(BIN)/*.* pattern
 
+# Generate version number record
+#
+$(BIN)/version.txt : $(MAKEDEPS)
+	@$(ECHO) "$(VERSION)" > $@
+
 # Analyse a target name (e.g. "bin/dfe538--prism2_pci.rom.tmp") and
 # derive the variables:
 #