kernel: Initial implementation of `TARGET_KERNEL_BUILD_THREADS`

* Utilizing `KERNEL_MAKE_FLAGS` variable in its core and working
  exactly the same way as if it's defined in
  `TARGET_KERNEL_ADDITIONAL_FLAGS`, this variable simply sets the
  threads used during kernel compilation, resulting in a way less
  time consumed for the kernel build process. It's optional to use
  this variable and it's defaulted to 1. The need for this change is
  basically coming from some mid-low-end machines that are (ab)used
  to build Android ROMs, such as a machine with 3rd gen Intel Core i3
  that took an entirety of 14 hours to clean build.

Test: `m bootimage`, kernel uses configured thread count and takes
      less time to build.
Signed-off-by: Beru Hinode <windowz414@1337.lgbt>