MFC a few things related to buildkernel:

• add the reinstallkernel target (does a make reinstall)
• prefer KERNCONF to KERNEL, but accept KERNEL anyway (with a pointer to KERNCONF, but not as annoying as on -current)
• use 'make clean' instead of 'config -r'
• add a NO_KERNELCLEAN option to avoid a clean. NOCLEAN is respected.
• buildkernel checks to see if a kernel name was supplied
• Allow changing the installed kernel name.. eg: make buildkernel KERNCONF=GENERIC INSTKERNNAME=kernel.GENERIC

These should largely be transparent and should not break anything
or anyone. It should enable consistant doc targets between -current
and RELENG_4 now.