[arch][arm] properly account for nonzero KERNEL_LOAD_OFFSET
[platform][zynq] add support for putting the base of the system in dram (1MB offset)

-Add a few global #defines: ZYNQ_CODE_IN_SDRAM and ZYNQ_SDRAM_INIT which
are used in a few places to initialize or not initialze dram and affect
the way the VM is brought up.
-Add zybo-dram-test project