rpi3: add proper target to compile uboot.env

To be able to compile uboot.env directly we must have a proper target
instead of the current '$(RPI3_UBOOT_ENV)'. Therefore we replace
'$(RPI3_UBOOT_ENV)' with u-boot-env instead which also lines up better
with the u-boot-env-clean variable.

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Tested-by: Joakim Bech <joakim.bech@linaro.org> (RPi3, RPi3+NFS)
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>