x86: Factor out common values in the link script

Define the reset base in config.mk so that it does not need to be calculated
twice in the link script. Also tidy up the START_16 and RESET_VEC_LOC values
to fit with this new approach.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/arch/x86/cpu/config.mk b/arch/x86/cpu/config.mk
index f7b01d3..84aeaf3 100644
--- a/arch/x86/cpu/config.mk
+++ b/arch/x86/cpu/config.mk
@@ -12,5 +12,6 @@
 # DO NOT MODIFY THE FOLLOWING UNLESS YOU REALLY KNOW WHAT YOU ARE DOING!
 LDPPFLAGS += -DRESET_SEG_START=0xffff0000
 LDPPFLAGS += -DRESET_SEG_SIZE=0x10000
-LDPPFLAGS += -DRESET_VEC_LOC=0xfff0
-LDPPFLAGS += -DSTART_16=0xf800
+LDPPFLAGS += -DRESET_VEC_LOC=0xfffffff0
+LDPPFLAGS += -DSTART_16=$(CONFIG_SYS_X86_START16)
+LDPPFLAGS += -DRESET_BASE="CONFIG_SYS_TEXT_BASE + (CONFIG_SYS_MONITOR_LEN - RESET_SEG_SIZE)"