busybox: ensure $(BUSYBOX_BUILD_CONFIG) is writable

If $(BUSYBOX_CONFIG_FILE) is read-only (eg. because Buildroot's
source dir is), the sed fixups to $(BUSYBOX_BUILD_CONFIG) fail.

Fixes: #4363
Signed-off-by: Nathan Lynch <ntl@pobox.com>
[yann.morin.1998@free.fr: use $(INSTALL) instead of cp]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>