[app][lkboot] complete boot command in a thread

so we can allow the lkboot transaction to complete and the
host tool doesn't hang waiting for an ack
[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
[kernel][vm] tweak the pmm_alloc_contiguous routine to take alignment in log2
[vm] when marking boot physical pages used, make sure the address range is inclusive
[platform][zynq] on DRAM base builds, make sure the sram arena is sized properly
[lib][unittest] add unittest framework and app