Set openssh as a PREFERRED_PROVIDER for sshd

Both dropbear and openssh are valid providers for sshd. As there is no
distro policy at the moment, we pick openssh in the local.conf.sample.

Note: https://bugzilla.yoctoproject.org/show_bug.cgi?id=6149

Signed-off-by: Ash Charles <ashcharles@gmail.com>
[linux-gumstix] KERNEL_IMAGETYPE is recipe-specific

Newer kernel use device tree + zImage rather than a uImage hence the
KERNEL_IMAGETYPE parameter is specific to the kernel recipe used rather
than particular machine.

Also specified the LINUX_VERSION and other little clean-ups.

Signed-off-by: Ash Charles <ashcharles@gmail.com>