initramfs: fix disabling of initramfs (and its compression)

Commit db2aa7fd ("initramfs: allow again choice of the embedded
initram compression algorithm") introduced the possibility to select the
initramfs compression algorithm from Kconfig and while this is a nice
feature it broke the use case described below.

Here is what my build system does:

 - kernel is initially configured not to have an initramfs included

 - build the user space root file system

 - re-configure the kernel to have an initramfs included
   (CONFIG_INITRAMFS_SOURCE="/path/to/romfs") and set relevant
   CONFIG_INITRAMFS options, in my case, no compression option
   (CONFIG_INITRAMFS_COMPRESSION_NONE)

 - kernel is re-built with these options -> kernel+initramfs image is
   copied

 - kernel is re-built again without these options -> kernel image is
   copied

Building a kernel without an initramfs means setting this option:

  CONFIG_INITRAMFS_SOURCE="" (and this one only)

whereas building a kernel with an initramfs means set...