binman: Support a list of strings with the mkimage etype

At present the 'args' property of the mkimage entry type is a string. This
makes it difficult to include CONFIG options in that property. In
particular, this does not work:

   args = "-n CONFIG_SYS_SOC -E"

since the preprocessor does not operate within strings, nor does this:

   args = "-n" CONFIG_SYS_SOC" "-E"

since the device tree compiler does not understand string concatenation.

With this new feature, we can do:

   args = "-n", CONFIG_SYS_SOC, "-E";

Signed-off-by: Simon Glass <sjg@chromium.org>