linux: don't force no initramfs

When initramfs was not selected as a root filesystem, we forcefully
disabled the initramfs in the kernel configuration.

However, it prevents an user from manually managing its initramfs, as
we override the option he has set in his kernel configuration
file. There's no real reason to do so: when initramfs is not selected
as the root filesystem, just don't touch initramfs related options in
the kernel configuration.

Problem reported by Sergey Naumov <sknaumov@gmail.com>.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>