Writing empty string gets ignored. This causes problems for example
when switching from mass-storage mode to charging only as the
lua/file attribute does not get cleared.

Use a wrapper function to append a linefeed to strings written to sysfs.
This avoids zero length writes and makes behavior similar to how
redirected echo from command line works.

Also tweak unexpected-sysfs-content-change tracker so that it ignores
changes in whitespace.
Signed-off-by: Simo Piiroinen <simo.piiroinen@jollamobile.com>