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