updater: Let read_file() return Value::Type::STRING.

It used to return a Value blob to be consumed by sha1_check() (which has
been deprecated). Currently there's no other generic updater function
that works with BLOB Values. This CL changes read_file() to return a
string Value to make it more useful (e.g. allowing equality check).

Test: Run recovery_component_test and recovery_unit_test on marlin.
Change-Id: Iba986ba649030112babefe898f26aa9ffe69eeb7