build: Rework version generation; don't allow make version override

Convert the script to generate the build version from a shell script
to a python script.

Remove the ability to override the version at build time via "make
VERSION=xyz".  Replace it with ability to add extra version
information at build time via "make EXTRAVERSION=xyz".

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/vgasrc/vgaversion.c b/vgasrc/vgaversion.c
new file mode 100644
index 0000000..02c8ea3
--- /dev/null
+++ b/vgasrc/vgaversion.c
@@ -0,0 +1,5 @@
+// Place build generated version into a C variable
+#include "autovgaversion.h"
+#include "types.h"
+
+char VERSION[] VAR16 = BUILD_VERSION;