Made kern a switching header file directory.

Made kern a switching header file directory.

SConstruct:
    Put the code to make a switching header directory into a function so they are easy to make.
src/arch/SConscript:
    Replace switching header code with the new function call.
src/kern/SConscript:
    Created a new switching header directory in kern, and moved the declaration of some source files here.

--HG--
rename : src/kern/kernel_stats.cc => src/kern/base_kernel_stats.cc
rename : src/kern/kernel_stats.hh => src/kern/base_kernel_stats.hh
extra : convert_revision : 98f5320a5ade567c3e4f67fef123dfb0c5122545