u-boot: Fix syntax error in ${UBOOT_ENV}.scr compilation

A previous commit (a3d3c2d4ac421a0dde2a20825eab4434f16b2452) introduced
support for compiling a U-Boot boot script, but the logic contained a
syntax error which was only visible in the build log. Fix the error by
using separate []s for each expression in the if statement.

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>