configs/imx6slevk: Bump U-Boot version

configs/imx6slevk: Bump U-Boot version

U-Boot 2017.11 has the following build issue:

  MKIMAGE u-boot-dtb.imx
Error: No BOOT_FROM tag in board/freescale/mx6slevk/imximage.cfg.cfgtmp
arch/arm/mach-imx/Makefile:86: recipe for target 'u-boot-dtb.imx' failed
make[2]: *** [u-boot-dtb.imx] Error 1
Makefile:907: recipe for target 'u-boot-dtb.imx' failed
make[1]: *** [u-boot-dtb.imx] Error 2
make[1]: *** Waiting for unfinished jobs....

This issue has been fixed by commit  f916757300c1  ("imx: Create
distinct pre-processed mkimage config files"), which landed in
U-Boot 2018.05.

Bump the U-Boot version to fix this build error.

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/136980040

Reported-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>