kernel: Fix module installation and clean up

 * Fold module installation into kernel image recipe.
 * Specify make argument to strip modules on installation instead of
   explicitly invoking the strip tool.
 * Remove support of TARGET_KERNEL_MODULES, it's a phony target thus
   we're not allowed to depend on it per Android 10 restriction. It
   has been useless for years anyway.
 * Apply minor clean-ups.

Change-Id: I288af1b2f741f524c2ef49554e15cb91c3115209