[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