Remove LOADER_ZFS_SUPPORT as a Makefile variable

LOADER_ZFS_SUPPORT is entirely used to select whether or not to
support ZFS in the loader. But it's not a user-servicable part (MK_ZFS
is what's used for that) Change it to the more conventional HAVE_ZFS
and move the ZFS support code into loader.mk. In addition, only build
ZFS libraries and boot loaders when ZFS is enabled.

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