[devmgr] Add new argv args for controlling driver loading

--driver-search-path specifies what path to search for driver DSOs
(defaulting to "/boot/driver").

--sys-device-driver specifies what path to open for the system
device driver (defaulting to "/boot/driver/bus-acpi.so" on x64 and
"/boot/driver/platform-bus.so" on arm64)

These will be used in the future for launching unprivileged devmgr
instances for the purposes of testing.

Test: Booted in qemu with and without each argument specified.

Change-Id: I8a2ddc299c7ddcd61b1d436bc3cc675488a927b1