Simplier way of building ONIE

Simplier way of building ONIE
Examples:

To run it, you can be either in its directory (ONIE/contrib/autobuild)
or in the ONIE directory.
$ ./build-onie.py -h
usage: build-onie.py [-h] [-l] [-b TARGET]

Build ONIE

optional arguments:
  -h, --help            show this help message and exit
  -l, --list            list all vendors and machines
  -b TARGET, --build TARGET
                        Vendor or Machine to compile

Using the list (-l or --list), you will see all possible vendors and
machines.
$ ./build-onie.py -l
UNKNOWN
        fsl_p2020rdbpca
        im_n29xx_t40n
        kvm_x86_64

accton
        accton_as4600_54t
        accton_as5600_52x
        accton_as5610_52x
        accton_as5710_54x
        accton_as5712_54x
        accton_as6700_32x
        accton_as6701_32x
        accton_as6710_32x
        accton_as6712_32x
        accton_as7700_32x
        accton_as7702_32x

alpha_networks
        alpha_networks_snq6070_320f
        alpha_networks_snq60a0_320f
        alpha_networks_snx6070_486f
        alpha_networks_snx60a0_486f

celestica
        cel_redstone_xp

dell
        dell_s4810_on_p2020
        dell_s6000_s1220

mellanox
        mlnx_x86

quanta
        quanta_lb8d
        quanta_lb9
        quanta_ly2
        quanta_ly3

Now, you can build any targets you want based on either the vendor or
machine names.

$ ./build-onie.py -b mlnx_x86
** Builds the mlnx_x86 machine under mellanox

$ ./build-onie.py -b accton
** Builds all machines under the accton vendor

$ ./build-onie.py -b quanta_lb9 -b cel_redstone_xp
** Builds both the quanta_lb9 and cel_redstone_xp machines

$ ./build-onie.py -b dell_s4810_on_p2020 -b alpha_networks
** Builds the dell_s4810_on_p2020 machine and all alpha_networks
machines

$ ./build-onie.py -b all
** Builds all machines and reference platforms
why do qemu targets have special processing?

probably better to fix up the qemu target's machine.make to "do the right thing" for the "make all" target instead.