[efi] Fix .efi and .efidrv linking

The linker chooses to look for _start first and always picks
efidrvprefix.o to satisfy it (probably because it's earlier in the
archive) which causes a multiple definition error when the linker
later has to pick efiprefix.o for other symbols.

Fix by using EFI-specific TGT_LD_FLAGS with an explicit entry point.

Signed-off-by: Piotr Jaroszyński <p.jaroszynski@gmail.com>
Signed-off-by: Joshua Oreman <oremanj@rwcr.net>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile.housekeeping

 #
 TGT_LD_FLAGS	= $(foreach SYM,$(TGT_LD_PREFIX) $(TGT_LD_DRIVERS) obj_config,\
 		    -u $(SYM) --defsym check_$(SYM)=$(SYM) ) \
-		  $(patsubst %,--defsym %,$(TGT_LD_IDS))
+		  $(patsubst %,--defsym %,$(TGT_LD_IDS)) \
+		  $(TGT_LD_FLAGS_PRE)
 
 # Calculate list of debugging versions of objects to be included in
 # the target.

src/arch/x86/Makefile.efi

 NON_AUTO_MEDIA	+= efi
 NON_AUTO_MEDIA	+= efidrv
 
+# Specify entry point
+#
+TGT_LD_FLAGS_PRE = -e _$(TGT_PREFIX)_start
+
 # Rules for building EFI files
 #
 $(BIN)/%.efi : $(BIN)/%.efi.tmp $(ELF2EFI)

src/arch/x86/prefix/efidrvprefix.c

  * @v systab		System table
  * @ret efirc		EFI return status code
  */
-EFI_STATUS EFIAPI _start ( EFI_HANDLE image_handle,
-			   EFI_SYSTEM_TABLE *systab ) {
+EFI_STATUS EFIAPI _efidrv_start ( EFI_HANDLE image_handle,
+				  EFI_SYSTEM_TABLE *systab ) {
 	EFI_STATUS efirc;
 
 	/* Initialise EFI environment */

src/arch/x86/prefix/efiprefix.c

  * @v systab		System table
  * @ret efirc		EFI return status code
  */
-EFI_STATUS EFIAPI _start ( EFI_HANDLE image_handle,
-			   EFI_SYSTEM_TABLE *systab ) {
+EFI_STATUS EFIAPI _efi_start ( EFI_HANDLE image_handle,
+			       EFI_SYSTEM_TABLE *systab ) {
 	EFI_STATUS efirc;
 
 	/* Initialise EFI environment */

src/arch/x86/scripts/efi.lds

  *
  */
 
-ENTRY ( _start )
-
 SECTIONS {
 
     /* The file starts at a virtual address of zero, and sections are