Convert CFG_SYS_UBOOT_START to Kconfig

Commit 65cc0e2a65 ("global: Move remaining CONFIG_SYS_* to CFG_SYS_*")
renamed CONFIG_SYS_UBOOT_START to CFG_SYS_UBOOT_START. Unfortunately,
this meant that the value was no longer available to the Makefile. This
caused imxrt to fail to boot. All the other boards that used this
variable were unaffected because they were using the default value
which is CONFIG_TEXT_BASE.

This commit converts CFG_SYS_UBOOT_START to Kconfig and sets the default
value to CONFIG_TEXT_BASE.

Suggested-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Suggested-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Jesse Taube <Mr.Bossman075@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

Kconfig

@@ -601,7 +601,27 @@ config TEXT_BASE
 	default 0x42e00000 if SUNXI_MINIMUM_DRAM_MB >= 64
 	hex "Text Base"
 	help
-	  The address in memory that U-Boot will be running from, initially.
+	  The address in memory that U-Boot will be copied and executed from
+	  initially.
+
+config HAVE_SYS_UBOOT_START
+	bool "Use custom U-Boot Start"
+	depends on HAVE_TEXT_BASE
+	help
+	  By default, the address in memory that U-Boot will be copied from
+	  (TEXT_BASE) and the entry point are the same. Select this to start the
+	  execution of U-Boot from a different address.
+	  This may be required if a header or vector table needs to be copied
+	  but not executed.
+
+config SYS_UBOOT_START
+	hex
+	depends on HAVE_TEXT_BASE
+	default TEXT_BASE
+	prompt "U-Boot entry" if HAVE_SYS_UBOOT_START
+	help
+	  If TEXT_BASE differs from the start of execution, this sets the
+	  address in memory that U-Boot will start execution from initially.
 
 config HAVE_SYS_MONITOR_BASE
 	bool