[profile] Provide methods for profiling individual stages of operations

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[librm] Add profiling self-tests for complete real_call and prot_call cycles

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[pxe] Profile all PXE API calls

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[pxe] Profile UNDI transmit datapath

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[pxe] Work around missing PXENV_UNDI_OPEN only when necessary

Signed-off-by: Michael Brown <mcb30@ipxe.org>