configs/imx6slevk: bump U-Boot
Bump U-Boot to version 2021.10 to fix the following build error:
/usr/bin/ld: scripts/dtc/dtc-parser.tab.o:(.bss+0x10): multiple definition of `yylloc'; scripts/dtc/dtc-lexer.lex.o:(.bss+0x0): first defined here
Fixes:
- https://gitlab.com/buildroot.org/buildroot/-/jobs/1915006177
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Fabio Estevam