Add optional salt to AUTOBOOT_STOP_STR_SHA256

Adds an optional SALT value to AUTOBOOT_STOP_STR_SHA256.   If a string
followed by a ":" is prepended to the sha256, the portion to the left
of the colon will be used as a salt and the password will be appended
to the salt before the sha256 is computed and compared.

Signed-off-by: Joel Peshkin <joel.peshkin@broadcom.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: Heiko Schocher <hs@denx.de>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Joel Peshkin <joel.peshkin@broadcom.com>
To: u-boot@lists.denx.de
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>