package/ca-certificates: fix rebuilds

package/ca-certificates: fix rebuilds

Rebuilding ca-certificates using make ca-certificates-rebuild
caused duplicate certificates to be installed in the target. Its build
system is broken: it doesn't detect that the output file already exists,
and instead of overwriting it, a duplicate is generated under a
different name. The net effect is that all certificates are installed
twice after rebuild.

Fix this by cleaning the build directory before building the package.

Signed-off-by: Martin Bark <martin@barkynet.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>