Fix building profiling kernels by passing the -p flag to config(8)
if the kernel's name ends in .PROF.

problem reported by jmc via mpi
ok mpi

Unbreak 'config -p'.

ok tb@, jca@

Two tweaks for compile/Makefile.inc:

1) Replace '.elif !exists(${OBJDIR}/Makefile)' with just '.else'. espie
pointed out, that if the file existed, make wouldn't be reading this
file, so the check is superflous. Less clutter.

2) Unconditionally define the 'clean' and 'cleandir' targets, also when
obj doesn't exist. This changes the behaviour of 'make clean' to be
successful (doing nothing) without obj@ or obj/.

ok tb millert deraadt

cleandir: target for kernel compile directories
ok natano

Kernel builds now happen in compile/CONFIG/obj@ -> /usr/obj/... [or ./obj/,
if srctree is not rooted at /usr/src]. As a result, stock GENERIC & RAMDISK
kernels are commited to the tree, to ensure the src tree can be "readonly"
during builds, with all writes occuring inside the obj space. config -b
options are handled by ../Makefile.inc. The canonical new way to configure
one of these kernels is:
% cd /sys/arch/amd64/compile/GENERIC.MP
% doas make obj
% make config
% make
% doas cp obj/bsd /bsd
The build infrastructure will use this new mechanism in a de-escalation
way using BUILDUSER.
Much help from natano and tb.