board: fsl_validate: Fix resource leak issue

Free dynamically allocated memory before every return statement
in calc_img_key_hash() and calc_esbchdr_esbc_hash() function.
Verified the secure boot changes using ls1046afrwy board.

Signed-off-by: Kshitiz Varshney <kshitiz.varshney@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>