Convert CFG_SYS_UBOOT_START to Kconfig

Commit 65cc0e2a ("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>