cmd: improve string matching for hex

Match the "=0x" instead of just "=0".

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Heinrich.Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>