eal: customize delay function

eal: customize delay function

When running single-core, some drivers tend to call rte_delay_us for a
long time, and that is causing packet drops.
To avoid this, rte_delay_us can be replaced with user-defined delay
function with:

void rte_delay_us_callback_register(void(*userfunc)(unsigned));

When userfunc==rte_delay_us_block build-in blocking delay function is
restored.

Signed-off-by: Jozef Martiniak <jozmarti@cisco.com>