kernel: avoid underscore in *6lowpan package names

Packages kmod-bluetooth_6lowpan and kmod-ieee802154_6lowpan contain an
underscore in the package name.  This causes problems in package/install
because when building a list of package files to install offline using
opkg, it uses a wildcard of the form $(dir)/$(pkg)_*.ipk.

If you were to select kmod-bluetooth=y, but kmod-bluetooth_6lowpan=m,
the latter would be picked up by that wildcard, and make package/install
would fail:

Collected errors:
 * satisfy_dependencies_for: Cannot satisfy the following dependencies
 * for kmod-bluetooth_6lowpan:
 *      kmod-6lowpan
 * opkg_install_cmd: Cannot install package kmod-bluetooth_6lowpan.

Changing the wildcard pattern is not trivial, and there may be other
places in the build system making this assumption about the package name
format.

Using a dash in place of the underscore avoids the issue.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>