uclibc: copy config file from configure iso patch step

The three typical packages that use .config files in buildroot copy the
config file at different times in the build process:

    busybox copies its .config from the post-extract hook.
    linux copies its .config in the configure_cmds.
    uclibc copies its .config from the post-patch hook.

Copying the .config file from the configure step is the only way to properly
support an OVERRIDE_SRCDIR that does not yet have the .config file, because
the extract and patch steps are skipped in that case.

In a previous patch, the situation was already fixed for busybox. This patch
applies the same fix to uclibc: copy the config file from the configure
step, as linux is doing.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>