kbuild: Support flat DTBs install

In preparation to move Arm .dts files into sub-directories grouped
by vendor/family, the current flat tree of DTBs generated by
dtbs_install needs to be maintained. Moving the installed DTBs to
sub-directories would break various consumers using 'make dtbs_install'.

This is a NOP until sub-directories are introduced.

Signed-off-by: Rob Herring <robh@kernel.org>