x86: doc: Update coreboot payload entry point address

With recent EFI support, the entry point address of coreboot payload
was changed. Now we update the address to use _x86boot_start, which
is the same one for EFI.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/doc/README.x86 b/doc/README.x86
index e7dc090..85ff181 100644
--- a/doc/README.x86
+++ b/doc/README.x86
@@ -245,10 +245,10 @@
 
 # in the coreboot root directory
 $ ./build/util/cbfstool/cbfstool build/coreboot.rom add-flat-binary \
-  -f u-boot-dtb.bin -n fallback/payload -c lzma -l 0x1110000 -e 0x1110015
+  -f u-boot-dtb.bin -n fallback/payload -c lzma -l 0x1110000 -e 0x1110000
 
-Make sure 0x1110000 matches CONFIG_SYS_TEXT_BASE and 0x1110015 matches the
-symbol address of _start (in arch/x86/cpu/start.S).
+Make sure 0x1110000 matches CONFIG_SYS_TEXT_BASE, which is the symbol address
+of _x86boot_start (in arch/x86/cpu/start.S).
 
 If you want to use ELF as the coreboot payload, change U-Boot configuration to
 use CONFIG_OF_EMBED instead of CONFIG_OF_SEPARATE.