felboot: Reduce to only a linkerscript + stubs to compensate for general SPL config

The stubs is only needed because we are using objects from a full
SPL build. Even these will go away when felboot is integrated in
u-boot, reducing it to only a link script and config changes.

felboot/fel-boot.ld

 OUTPUT_FORMAT("elf32-littlearm", "elf32-littlearm", "elf32-littlearm")
 OUTPUT_ARCH(arm)
-ENTRY(_start)
+ENTRY(s_init)
 SECTIONS
 {
  . = 0x00002000;
  . = ALIGN(4);
  .text :
  {
-  main.o (.text.start)
+  *(.text.s_init)
   *(.text*)
  }
  . = ALIGN(4);

felboot/main.c

@@ -18,12 +18,6 @@
  */
 
 #include <common.h>
-#include <version.h>
-
-__attribute__ ((section (".text.start"))) void _start(void)
-{
-	s_init();
-}
 
 int sunxi_mmc_init(void)
 {