kernel: Ensure that module load lists are properly formed

Do not allow module load lists to contain unexistent kernel modules
and bail out if that's the case, informing that the list must be
corrected.

The lists can be formed including the full module path and with or
without the file extension, like shown below:

  kernel/drivers/watchdog/softdog.ko
  kernel/net/wireless/cfg80211.ko
  kernel/net/mac80211/mac80211.ko

or simply:

  softdog.ko
  cfg80211.ko
  mac80211.ko

or ultimately:

  softdog
  cfg80211
  mac80211

Either way, the newly generated modules.load file will now only
contain the module name without the extension, which is inline with
modules.alias and modules.blocklist.

Change-Id: I969274f7edf249c98ca241e37b6e41a921d36908