fs: use our own mkpasswd when creating users

Export PATH=$(BR_PATH) so that our own mkpasswd is found before
any existing host-system one.

Reported-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Tested-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>