[pxe] Enable interrupts before starting PXE NBP execution

Based on a patch provided by XenSource for Etherboot 5.4.

src/arch/i386/interface/pxe/pxe_call.c

 	__asm__ __volatile__ ( REAL_CODE ( "pushw %%cx\n\t"
 					   "pushw %%ax\n\t"
 					   "movw %%cx, %%es\n\t"
+					   "sti\n\t"
 					   "lcall $0, $0x7c00\n\t"
 					   "addw $4, %%sp\n\t" )
 			       : "=a" ( rc ), "=b" ( discard_b ),