[install.sh] Install failed with ONIE 2017.11

[roberthong-qct]

Description

The messages when installing a SONiC binary are shown below:

Installing SONiC in ONIE
ONIE Installer: platform: x86_64-broadcom-r0
/tmp/tmp.p2OaXD/installer/install.sh: .: line 1: can't open '/etc/machine-build.conf='
Failure: Unable to install image: ftp://172.18.0.161/sonic-broadcom.bin

Steps to reproduce the issue:

1. Install a SONiC image with ONIE 2017.11
2. Get failing messages

Describe the results you received:

The "read_config_file" which introduced in #6600
reads /etc/machine.conf to get var and value pairs.
However for ONIE 2017.11, /etc/machine.conf is a script that needs to be executed.

Describe the results you expected:

Install.sh should handle different types of /etc/machine.conf in ONIE