mambo: Get the zImage from an environment variable

Rather than hard coding /tmp/zImage.epapr, get the zImage from an environment
variable.

This is a) cleaner, and b) allows running multiple copies of the
simulator against different kernels simultaneously.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Makefile: Support CROSS_COMPILE as well as CROSS

A lot of projects use CROSS_COMPILE for specifying the cross compile
prefix, so support that as well as CROSS.

For example this allows a user to set CROSS_COMPILE once in their
environment and build both Linux & Skiboot with the same setting.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
[stewart@linux.vnet.ibm.com: only set if env variable is set]
Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>