Remove residual blank line at start of Makefile

This is a residual of the $FreeBSD$ removal.

MFC After: 3 days (though I'll just run the command on the branches)
Sponsored by: Netflix

genet: add module (if_genet.ko)

The driver already had the appropriate module macros, it just wasn't
hooked into the build system.

Since this device is specific to the Raspberry Pi 4, only build it

genet: add module (if_genet.ko)

The driver already had the appropriate module macros, it just wasn't
hooked into the build system.

Since this device is specific to the Raspberry Pi 4, only build it for
AArch64.

Reviewed by: imp, emaste (earlier version)
Pull Request: https://github.com/freebsd/freebsd-src/pull/1139

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