libs/utils/env: ensure "/" is always mounted RW
libs/utils/env: ensure "/" is always mounted RW

This is a sanity check mainly required for ChromeOS targets but that in
general it's worth to be done for each target.

Signed-off-by: Patrick Bellasi <431c86f368affbc3148c4669fbd465131f7e82cb@arm.com>

ARM-software/lisa,joelagnel/lisa,credp/lisa,arnoldlu/lisa,joelagnel/lisa,bjackman/lisa,arnoldlu/lisa,credp/lisa,credp/lisa,ARM-software/lisa,mdigiorgio/lisa,ARM-software/lisa,JaviMerino/lisa,bjackman/lisa,mdigiorgio/lisa,ARM-software/lisa,credp/lisa