run: adjust network lwip test for x86
run: lwip use is_qemu
run: support RUN_OPT for all platforms

Use RUN_OPT="--target ..." to select the backend test method.

Supported values so far:

qemu     - qemu + grub bootloader (default)
qemu+pxe - qemu + pulsar bootloader (PXE)
amt      - Intel AMT (reset+serial output) + pulsar bootloader

Related to issue #598