x86: Add an option to control the position of U-Boot

The existing work-around for positioning U-Boot in the ROM when it
actually runs from RAM still exists and there is not obvious way to change
this.

Add a proper Kconfig option to handle this case. This also adds a new bool
property to indicate whether CONFIG_SYS_TEXT_BASE exists.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/configs/chromebook_samus_tpl_defconfig b/configs/chromebook_samus_tpl_defconfig
index 5ef2ecb..74b7e9a 100644
--- a/configs/chromebook_samus_tpl_defconfig
+++ b/configs/chromebook_samus_tpl_defconfig
@@ -16,6 +16,7 @@
 CONFIG_SMP=y
 CONFIG_HAVE_VGA_BIOS=y
 CONFIG_SPL_TEXT_BASE=0xffe70000
+CONFIG_X86_OFFSET_U_BOOT=0xfff00000
 CONFIG_BOOTSTAGE=y
 CONFIG_BOOTSTAGE_REPORT=y
 CONFIG_SHOW_BOOT_PROGRESS=y