linux-dummy: bundle_initramfs should not be nostamp any more

The same change has already been applied to kernel.bbclass in commit 55989cb5:
[ kernel.bbclass: Stop bundle_initramfs thwarting sstate cache and fix race ]

The dummy kernel should comply with it.

Signed-off-by: Ming Liu <ming.liu@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>