kmodloader: add basic support for builtin modules

kmodloader: add basic support for builtin modules

Enable parsing of 'modules.builtin' if present, for improved handling of
loadable and builtin modules similar to most other distros. In particular,
this allows 'modprobe' to return success whether a module is loadable or
built-in, a useful feature for consistent presence-testing. For example:

  root@OpenWrt:/# rmmod sch_fq_codel
  module is builtin

  root@OpenWrt:/# modinfo sch_cake
  filename:       /lib/modules/6.1.65/sch_cake.ko
  license:        Dual BSD/GPL
  depends:
  intree:         Y
  name:           sch_cake
  vermagic:       6.1.65 SMP mod_unload MIPS32_R2 32BIT

  root@OpenWrt:/# modprobe sch_fq_codel && echo SUCCESS || echo FAIL
  SUCCESS
  root@OpenWrt:/# modprobe sch_cake && echo SUCCESS || echo FAIL
  SUCCESS

Signed-off-by: Tony Ambardar <itugrok@yahoo.com>