This is a follow-up to 8f8ec56052766aa5105d672b77ad9eaca5c1ab3c, which only
covers building initrd, while the booting code still tries to load nonexistent
files for builtin modules.
* gnu/build/linux-modules.scm (load-linux-modules-from-directory): Ignore
nonexistent module files.
Change-Id: I09ef207e82397e915e671c8464b92bcf90f03dcf