docs: update github pull request instructions

Include additional details about using a topic branch when making a
pull request, instead of using the master branch.
docs: update FAQ regarding ONIE-izing old hardware

Update the FAQ for the commonly asked question: 

  I have a switch from vendor XYZ that does not have ONIE installed.
  Can I install ONIE on it myself?

Short answer: no
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] [-n] [-m ARGS]

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
  -n, --dry-run         perform a dry run, do not execute
  -m ARGS, --make-args ARGS
                        additional args to be passed to make

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

Update build-onie.

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>
xtools: restart build on failure

Today the crosstools-ng build has the bad property that when
restarting a build after a failure the crosstool-ng system believes
the toolchain is built correctly.  This leads to all manner of
downstream build errors and confusion for developers who are trying to
make progress after a crosstool-ng failure.

This patch coaxes the xtools target into being "out of date" whenever
the crosstool-ng build fails.  This is accomplished by removing the
$(XTOOLS_BUILD_DIR)/.config.2 file, which the crosstool-ng system uses
as a prerequisite for building, whenever the crosstool-ng build fails.

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>
x86: include the recovery ISO into the 'all' Makefile target

Reviewed By: Carlos Cardenas

When building the x86 based machine types it is often useful to have
the recovery ISO, though not strictly required all the time.

The build time of the recovery ISO is small compared to everything
else, so including it by default in the 'all' Makefile target has
little impact.

This patch adds building the recovery ISO to the 'all' Makefile target
for x86 machine.

Testing Done:

Compiled the kvm_x86_64 machine using the 'all' target.  Verified the
build was successful, the recovery ISO was created and nothing broke.

Compiled the dell/dell_s6000_s1220 machine using the 'all' target.
This machine is noteworthy because it's machine.make *already*
includes the building of the pxe-efi64 recovery image, which has the
recovery ISO as a dependency.  Verified the build was successful, the
recovery ISO was created and nothing broke.

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>
Merge remote-tracking branch 'upstream/master'