Add pre and post install hooks

Some platforms may have hardware watchdog enabled
in bios, and with facility to allow software to
set 'next-boot-device'.

The pre and post install hooks added here can
be used for platform specific routines, such as
reset hardware watchdog, set next-boot-device
in bios after successful onie installation.

Testing Done:

Tested on x86_64 platforms.
Add check for return code for pre and post install hooks.

Testing Done:

Tested on x86_64 platforms.
Remove hardcode console setting by onie installer

Use "GRUB_SERIAL_COMMAND" and "GRUB_CMDLINE_LINUX" if
defined, otherwise use default settings.
Remove hardcode of console unit and console speed

This patch allows setting console unit number and console-speed
at image build time. if CONSOLE_UNIT and CONSOLE_SPEED is not
defined, default is 0 and 115200.

This can be set as part of the make command (i.e,
make MACHINE=kvm_x86_64 CONSOLE_SPEED=9600 CONSOLE_UNIT=0 all),
or define in the machine.make file such as,

$ cat machine/kvm_x86_64/machine.make
CONSOLE_SPEED = 9600
CONSOLE_UNIT = 0
Merge branch '2014.05br' of https://github.com/sustephen/onie into 2014.05br