arm: Introduce MK_KERNEL_BIN to control generation of kernel.bin

It's sometimes desirable to generate kernel.bin and install it. While
the mainstream has moved on to UEFI booting on arm, some specialized
gear can't support it. For that gear, we unconditionally generate
kernel.bin. Add a knob so that WITH_KERNEL_BIN or WITHOUT_KERNEL_BIN
control its generation and installation. config files should add
'makeoptions WITH_KERNEL_BIN=t' to enable it. Since its use is
specialized, it is off by default now since the arm world has largely
moved on to UEFI.

It only affects arm and arm64 (since those are the only two that support
it).

Sponsored by: Netflix
Reviewed by: mmel
Differential Revision: https://reviews.freebsd.org/D39013