linux: ensure buildroot_defconfig is writable

If $(KERNEL_SOURCE_CONFIG) is read-only (eg. because Buildroot's source
dir is), the rm of $(KERNEL_ARCH_PATH)/configs/buildroot_defconfig will
either fail, or prompt the user, both of which we want to avoid.

Make it writable by using $(INSTALL).

Fixes: #4363
Signed-off-by: Nathan Lynch <ntl@pobox.com>
[yann.morin.1998@free.fr: use $(INSTALL) instead of cp, don't 'rm -f']
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>