radvd: use <pkg>_INSTALL_INIT_SYSV

This commit changes the radvd to use the <pkg>_INSTALL_INIT_SYSV
mechanism instead of using a post install target hook. We also use a
full destination path when calling $(INSTALL).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>