linux-boot: Load modules and their dependencies, à la 'modprobe'.
* gnu/build/linux-boot.scm: Use (gnu build linux-modules).
(load-linux-module*): Remove.
(boot-system): Add #:linux-module-directory parameter.
[lookup-module]: New procedure.
Call 'current-module-debugging-port'. Pass #:lookup-module to
'load-linux-module*'. Map LOOKUP-MODULE on LINUX-MODULES.
* gnu/system/linux-initrd.scm (base-initrd): Adjust 'boot-system' call
accordingly. Adjust #:modules argument as well.
* gnu/system.scm (operating-system-activation-script)[%modules]: Likewise.
* gnu/system/vm.scm (expression->derivation-in-linux-vm): Likewise.