boards: xtensa: mention the $HOME/.flexlmrc file alternative

While environment variables are generally evil, we had a especially
unreliable CI experience with `XTENSAD_LICENSE_FILE` that went away when
we switched to `$HOME/.flexlmrc`. One of the things I observed: the
content of `XTENSAD_LICENSE_FILE` gets automatically added to
`$HOME/.flexlmrc`. This raises "fun" concurrency issues when compiling
with a lot of threads!

Signed-off-by: Marc Herbert <marc.herbert@intel.com>