bsd.port.mk: set SPLIT_KERNEL_DEBUG off

[emaste]

Summary

Base e3709cfe6a2a introduced a SPLIT_KERNEL_DEBUG knob to control
whether kernel debug is placed in standalone debug files at build time,
restoring the KERNEL_SYMBOLS knob to its historical behaviour of
controlling only whether or not the .debug files are installed.

Follow suit in ports and set SPLIT_KERNEL_DEBUG off.  We still set
KERNEL_SYMBOLS off for backwards compatibility.  This could be removed
once all supported base system versions have SPLIT_KERNEL_DEBUG.

Sponsored by:   The FreeBSD Foundation

[emaste]

src repo link: rG:e3709cfe6a2a