Fix parsing of shell command output regardless of encoding

We always know the start and end control patterns will be in UTF8.
However, the content in between could be encoded differently.

Search for the patterns in the shell command output using raw bytes,
instead of a string pattern match.

LETTUCE-755
Change-Id: I6cb019b644d371c37593fe26784f6e9d41844dec
Signed-off-by: Roman Birg <roman@cyngn.com>
(cherry picked from commit 91c9e78c)