switch i386 memstick installer images to MBR

Some BIOSes have trouble booting from GPT in non-UEFI mode. This is
commonly reported with Lenovo laptops, including my x220. As we do not
currently support booting FreeBSD/i386 via UEFI there's no reason to
prefer GPT.

The "vestigial swap partition" was added in r265017 to work around an
issue with loader's GPT support, so we should not need it when using
MBR.

We may want to make the same change to amd64, although the issue there is
mitigated by such systems booting via UEFI in the common case.

PR: 227422
Reviewed by: gjb
MFC after: 3 weeks
Relnotes: Yes
Sponsored by: The FreeBSD Foundation