u-boot: Convert ${UBOOT_ENV}.cmd into ${UBOOT_ENV}.scr

 * Add extra SRC variables to uboot-config.class for source cmd file
 * Add DEPENDS on u-boot-mkimage-native if UBOOT_ENV_SUFFIX is scr
 * Compile cmd -> scr in do_compile if UBOOT_ENV_SUFFIX is scr

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>