linux: fix linux-% shortcut targets

linux-% shortcut targets (short for linux26-%) ignores the ouput dir
$(O) so that 'make O=output.arm linux-menuconfig' is actually run in the
default $(O) directory output/ and not in output.arm/. Fix by passing on
$(O) if set.

[Peter: Use EXTRAMAKEARGS]
Signed-off-by: Bjørn Forsman <bjorn.forsman@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>