package/busybox: update minimal configuration file

package/busybox: update minimal configuration file

Commit 44a563dbc04ec8e51c5262201cd1745617055b78 bumps busybox to version
1.26.0, but does not update the minimal configuration file. There is at
least one issue using the old configuration with the newer busybox:

* IFUPDOWN is split into IFUP and IFDOWN in version 1.26.0

Update the minimal configuration file by loading the busybox.config file
and saving it back.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>