The imxrt1064, imxrt1064, and teensy-4.x board configs would fail to
build when CONFIG_USERLED_LOWER was enabled, due to a missing header in
imxrt_appinit.c and an unset local define.
Verified a build succeds for the specified boards with the following
defconfig modifications:
# CONFIG_ARCH_LEDS is not set
CONFIG_USERLED=y
CONFIG_USERLED_LOWER=y
Tested the "leds" Led driver example on a Teensy 4.1.
