sysutils/u-boot-master etc.: u-boot Improvement for 2018.07

• Add metadata for IMX6 family
• While here rename the riotboard family to IMX6 as it should have been
• Remove the cache patches, they aren't needed anymore. I should have checked that with the 2018.07 update
• beaglebone use a custom bootcmd that calls envboot, this command will try to load and source the file boot.scr (u-boot script) from the mmc. For some reason it seems to corrupts some memory or something because loading and executing ubldr.bin after running this command will either hang or cause an exception on undefined instruction. I'll check with upstream but in the meantime use a special fragment that redefine the bootcmd. Note that since u-boot will load the environment from the eMMC, one need to 'env default -f -a; saveenv' to have the bootcmd variable correctly redefined.

Reviewed by: imp
Approved by: imp
Differential Revision: https://reviews.freebsd.org/D16599