buildroot

packages: remove unneeded _INSTALL_TARGET_OPT definitions

Now that <pkg>_INSTALL_TARGET_OPT always defaults to
'DESTDIR=$(TARGET_DIR) install', we can remove the
<pkg>_INSTALL_TARGET_OPT definition from a lot of packages.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>