arm: exynos5: Migrate USB_BOOTING to Kconfig

This symbol is enabled for all exynos5 platforms, move to Kconfig and
select it.

Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Signed-off-by: Tom Rini <trini@konsulko.com>