[pci] Use linker tables for runtime selectable PCI APIs

Use the linker table mechanism to enumerate the underlying PCI I/O
APIs, to allow PCIAPI_CLOUD to become architecture-independent code.

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

src/config/cloud/ioapi.h

@@ -4,4 +4,7 @@
 #ifdef PLATFORM_pcbios
 #undef PCIAPI_PCBIOS
 #define PCIAPI_CLOUD
+#define PCIAPI_RUNTIME_ECAM
+#define PCIAPI_RUNTIME_PCBIOS
+#define PCIAPI_RUNTIME_DIRECT
 #endif

src/config/config_pci.c

@@ -22,6 +22,7 @@
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 #include <config/settings.h>
+#include <config/ioapi.h>
 
 /** @file
  *
@@ -34,3 +35,13 @@ PROVIDE_REQUIRING_SYMBOL();
 #ifdef PCI_SETTINGS
 REQUIRE_OBJECT ( pci_settings );
 #endif
+
+#ifdef PCIAPI_RUNTIME_ECAM
+REQUIRE_OBJECT ( ecam );
+#endif
+#ifdef PCIAPI_RUNTIME_PCBIOS
+REQUIRE_OBJECT ( pcibios );
+#endif
+#ifdef PCIAPI_RUNTIME_DIRECT
+REQUIRE_OBJECT ( pcidirect );
+#endif