bootm: Remove BOOTM_NETBSD from defaults

bootm: Remove BOOTM_NETBSD from defaults

After talking with someone from the NetBSD project, platforms that do
not boot with a device tree (and so would be using our BOOTM_NETBSD
support) a very few in number. So we can remove this option from being
enabled by default and save a little space in most places with platforms
that need it still being able to re-enable it, if needed. Ideally, in a
few years we can instead just remove the code entirely.

Link: https://lore.kernel.org/r/aWKQOW_ajq0DsbYA@big-apple.aprisoft.de/
Signed-off-by: Tom Rini <trini@konsulko.com>