When sharing a common build directory between different build platforms
eg. macos v docker based linux v virtual machine, a 'make dirclean'
isn't quite enough to clean all the platform related binaries.  The
'conf' and 'mconf' aka 'make menuconfig/defconfig & friends' utilities
remain.

Clean those as part of 'dirclean' so they get rebuilt for the current
platform on the next 'make'

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>