MLBEDSW-6593: Issue with finding some config files

The argument to the lstrip function is a list of all characters that
should be stripped from the beginning of the string, in any order. To
remove the actual prefix, check if the string starts with the string
instead and then remove that amount of characters. The function
"removeprefix" was added in python3.9 which does exactly this, but
that is not yet available to vela since it supports python 3.7.

Signed-off-by: Rickard Bolin <rickard.bolin@arm.com>
Change-Id: Ibc5a173c6d422cb5f55feb80caef6c5c30cf7d39