toolchain: instrument external toolchain wrapper

If BR_DEBUG_WRAPPER is set in the envirnment, dump the actual command
being exec()uted, to ease debugging issues with the wrapper.

[Peter: adjust code style and output format to be cut'n'paste compatible]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>