package/pkg-generic.mk: use more sensible default value for <pkg>_CPE_ID_UPDATE

Setting the "update" field in the CPE ID to * doesn't actually make
much sense, as * is a wildcard. Instead, this field should really
reflect the "subrelease" / "update" of the package, which unless
specified explicitly by the package .mk file, is empty.

Using a wildcard causes a few CVEs to be incorrectly identified as
affecting some of our packages.

For example https://nvd.nist.gov/vuln/detail/CVE-2013-1428 has a
CPE configuration that goes like this:

cpe:2.3🅰️tinc-vpn:tinc:*:pre6:*:*:*:*:*:* up to including 1.1

and this CPE configuration is currently identified as affecting our
package. This isn't correct as our package is using 1.0.36, not a
"pre6" version. But because the CPE ID generated by Buildroot uses *
as the "update" field, and * is the wildcard, it does match with this
CPE configuration.

After this change, two CVEs are no longer identified as affecting
Buildroot packages:

  https://nvd.nist.gov/vuln/detail/CVE-2013-1428
  https://nvd.nist.gov/vuln/detail/CVE-2017-9454

and in both cases they are indeed CVEs not affecting our package.

Reported-by: Titouan Christophe <titouan.christophe@mind.be>
Cc: Titouan Christophe <titouan.christophe@mind.be>
Cc: Thomas Perale <thomas.perale@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Titouan Christophe <titouan.christophe@mind.be>
Signed-off-by: Arnout Vandecappelle <arnout@rnout.be>

package/pkg-generic.mk

@@ -727,8 +727,6 @@ ifeq ($$($(2)_CPE_ID_VALID),YES)
  ifndef $(2)_CPE_ID_UPDATE
   ifdef $(3)_CPE_ID_UPDATE
    $(2)_CPE_ID_UPDATE = $$($(3)_CPE_ID_UPDATE)
-  else
-   $(2)_CPE_ID_UPDATE = *
   endif
  endif