u-boot: Update to v2018.07

This add:

• Explicit Depency on bison
• Remove the DTC= on MAKE_ARGS as we use the BINARY_ALIAS already
• metadata variables
• Add cache cmds and disable secure boot
• All RPI uses some special fragments to enable OF_BOARD. This makes u-boot using the DTB loaded by the firmware instead of using it's own.
• For patches accepted but not in mainline due to timing issue use patchwork directly instead of putting files in the port
• All armv7 boards are EFI enabled and boot in secure mode

More on metadata file :

The goal is to have a u-boot-install package that will read those and auto
install u-boot either on a dir (method copy) or on a device (method raw)
As all the u-boots are either one file to be dd'ed or one/multiple files
to be copied this simple method is good enough.
Since the maximum variable is 4 (for the raw method), simply use echo instead
of a .in files (ok bapt@)

Reviewed by: imp
Approved by: imp (implicit)
Differential Revision: https://reviews.freebsd.org/D16241