Consider top-level targets to be .PHONY as bmake won't build them

otherwise if a file with the same name is found in the directory.

MFC after:	1 week
Sponsored by:	EMC / Isilon Storage Division

Makefile

@@ -250,7 +250,7 @@ ${TGTS}: .MAKE
 tinderbox toolchains kernel-toolchains: .MAKE
 .endif
 
-${TGTS}:
+${TGTS}: .PHONY
 	${_+_}@cd ${.CURDIR}; ${_MAKE} ${.TARGET}
 
 # The historic default "all" target creates files which may cause stale

Makefile.inc1

@@ -1062,7 +1062,7 @@ INSTALLKERNEL= ${_kernel}
 .endif
 .endfor
 
-buildkernel ${WMAKE_TGTS:N_worldtmp} ${.ALLTARGETS:M_*:N_worldtmp}: .MAKE
+buildkernel ${WMAKE_TGTS:N_worldtmp} ${.ALLTARGETS:M_*:N_worldtmp}: .MAKE .PHONY
 
 #
 # buildkernel