[mips] Improve MIPS trampoline code

This patch fix trampoline build. inckern.S increases stack and calls _startC
of elf_trampoline, so inckern.S should be called before elf_trampoline. gcc4.2 puts
text (code) into image according to order of source files in this call, so order has changed.

In addition make will install trampoline kernel. It allows to use kernel for firmware build.

Submitted by: Michael Zhilin <mizhka@gmail.com>
Differential Revision: https://reviews.freebsd.org/D6242