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