uboot: When Mender client writes U-Boot environment with fw_setenv, it results in an empty value instead of the real value, bricking devices.

Problem:

• The problem stems from this commit.
• While the comment in the commit is correct, the problem is that U-Boot has not supported this syntax for very long, so it still breaks on older U-Boot forks.

Preferred fix:

• Strictly speaking, it is libubootenv which broke this, not us.
• Fix libubootenv so that it supports variable value syntax, in addition to variable=value.
• Submit upstream.
• While waiting for acceptance, submit to meta-mender/master and meta-mender/dunfell as a libubootenv.bbappend patch.
• Revert the Mender client commit.

Backup fix (more hacky, less desirable):

• Adapt Mender client so that it first writes a dummy variable using the new syntax, checks that to see if it can read it back, and then uses that to decide which syntax to use.