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>
(cherry picked from commit 42b10634c628918c753bfb1aad4f950fa5d41299)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/ca-certificates/ca-certificates.mk

@@ -16,7 +16,7 @@ CA_CERTIFICATES_LICENSE = GPL-2.0+ (script), MPL-2.0 (data)
 CA_CERTIFICATES_LICENSE_FILES = debian/copyright
 
 define CA_CERTIFICATES_BUILD_CMDS
-	$(TARGET_MAKE_ENV) $(MAKE) -C $(@D) all
+	$(TARGET_MAKE_ENV) $(MAKE) -C $(@D) clean all
 endef
 
 define CA_CERTIFICATES_INSTALL_TARGET_CMDS