intel-microcode: fix build error on rebuild

intel-microcode: fix build error on rebuild

When rebuilding the package, iucode_tool fails with a "File exists"
error if the directory already contained firmware from the previous
build (incl. in the successful build case).

Fix this by removing the directory before invoking iucode_tool.

Fixes the following build error:
iucode_tool: 06-0f-02: cannot write to, or create file: File exists
make[2]: *** [Makefile:57: .../.built] Error 2

Signed-off-by: Rany Hany <rany_hany@riseup.net>
Link: https://github.com/openwrt/openwrt/pull/24077
Signed-off-by: Robert Marko <robimarko@gmail.com>