[pxe] Make pxe_init_structures() an initialisation function

pxe_init_structures() fills in the fields of the !PXE and PXENV+
structures that aren't known until gPXE starts up.  Once gPXE is
started, these values will never change.

Make pxe_init_structures() an initialisation function so that PXE
users don't have to worry about calling it.

src/arch/i386/image/pxe_image.c

 	int rc;
 
 	/* Ensure that PXE stack is ready to use */
-	pxe_init_structures();
 	pxe_hook_int1a();
 
 	/* Arbitrarily pick the most recently opened network device */

src/arch/i386/include/pxe_call.h

 
 extern void pxe_hook_int1a ( void );
 extern int pxe_unhook_int1a ( void );
-extern void pxe_init_structures ( void );
 extern int pxe_start_nbp ( void );
 extern __asmcall void pxe_api_call ( struct i386_all_regs *ix86 );
 

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

 FILE_LICENCE ( GPL2_OR_LATER );
 
 #include <gpxe/uaccess.h>
+#include <gpxe/init.h>
 #include <registers.h>
 #include <biosint.h>
 #include <pxe.h>
  * Initialise !PXE and PXENV+ structures
  *
  */
-void pxe_init_structures ( void ) {
+static void pxe_init_structures ( void ) {
 	uint32_t rm_cs_phys = ( rm_cs << 4 );
 	uint32_t rm_ds_phys = ( rm_ds << 4 );
 
 	pxenv.Checksum -= pxe_checksum ( &pxenv, sizeof ( pxenv ) );
 }
 
+/** PXE structure initialiser */
+struct init_fn pxe_init_fn __init_fn ( INIT_NORMAL ) = {
+	.initialise = pxe_init_structures,
+};
+
 /**
  * Start PXE NBP at 0000:7c00
  *

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

 	DBG ( "[PXENV_UNDI_LOADER to CS %04x DS %04x]",
 	      undi_loader->UNDI_CS, undi_loader->UNDI_DS );
 
-	/* Set up PXE data structures */
-	pxe_init_structures();
-
 	/* Fill in UNDI loader structure */
 	undi_loader->PXEptr.segment = rm_cs;
 	undi_loader->PXEptr.offset = __from_text16 ( &ppxe );