kconfig infra: make update-config/defconfig handling symmetrical

Replace a 'cp --preserve' with a 'touch --reference' so that the code
for foo-update-config and foo-update-defconfig is symmetrical to ease
maintainability and increase clarity.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: "Yann E. Morin" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>