ask to map in oxpcie i/o memory and support serial i/o for it in the
kernel. set oxpcie=<address> in boot monitor (retrieve address using
pci_debug=1 output). (no sanity checking is done on the address
currently.) disabled by default.
The change also contains some other minor cleanup (a new serial.h to set
register info common to UART and the OXPCIe card, in-kernel memory
mapping a little more structured and env_get() to get sysenv variables
without knowing about the params_buffer).
