mkimage: use environment variable MKIMAGE_SIGN_PIN to set pin for OpenSSL Engine

This patch adds the possibility to pass the PIN the OpenSSL Engine
used during signing via the environment variable MKIMAGE_SIGN_PIN.
This follows the approach used during kernel module
signing ("KBUILD_SIGN_PIN") or UBIFS image
signing ("MKIMAGE_SIGN_PIN").

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>