arm: imx: hab: Make authenticate_image return int

Both usages of authenticate_image treat the result code as a simple binary.
The command line usage of authenticate_image directly returns the result
code of authenticate_image as a success/failure code.

Right now when calling hab_auth_img and test the result code in a shell a
passing hab_auth_img will appear to the shell as a fail.

The first step in fixing this behaviour is to fix-up the result code return
by authenticate_image() itself, subsequent patches fix the interpretation
of authenticate_image so that zero will return CMD_RET_SUCCESS and non-zero
will return CMD_RET_FAILURE.

The first step is fixing the return type in authenticate_image() so do that
now.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Sven Ebenfeld <sven.ebenfeld@gmail.com>
Cc: George McCollister <george.mccollister@gmail.com>
Cc: Breno Matheus Lima <brenomatheus@gmail.com>
Tested-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>