*.mk: replace (TARGET|HOST)_PATH by BR_PATH

Thanks to the 2 previous patches of the series, BR_PATH contains
all locations in which host-packages may install programs.

This patch replaces the occurrences TARGET_PATH and HOST_PATH with
BR_PATH, everywhere these variables are used in the *.mk files.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>