boot: efi: Use an event to relocate the OS

Rather than a weak function, use the recently added event to adjust the
OS load-address. Tidy up the code a little while here.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/bootm.h

@@ -263,12 +263,6 @@ void arch_preboot_os(void);
  */
 void board_quiesce_devices(void);
 
-/*
- * boards should define this if they need to fix up the kernel before boot
- * (eg, by modifying the desired load address).
- */
-void board_fixup_os(struct image_info *os);
-
 /**
  * switch_to_non_secure_mode() - switch to non-secure mode
  */