mkconfig: add support for SPL CPU

Add support for specifying a differnt CPU for main u-boot and SPL
u-boot builds.  This is done by adding an optional SPL CPU after the
main CPU in boards.cfg as follows:

     normal_cpu:spl_cpu

This this case CPU will be set to "normal_cpu" during the main u-boot
build and "spl_cpu" during the SPL build.

Signed-off-by: Allen Martin <amartin@nvidia.com>
diff --git a/boards.cfg b/boards.cfg
index 0598763..becf258 100644
--- a/boards.cfg
+++ b/boards.cfg
@@ -11,6 +11,11 @@
 #	Lines starting with '#' are comments.
 #	Blank lines are ignored.
 #
+#	The CPU field takes the form:
+#		cpu[:spl_cpu]
+#	If spl_cpu is specified the make variable CPU will be set to this
+#	during the SPL build.
+#
 #	The options field takes the form:
 #		<board config name>[:comma separated config options]
 #	Each config option has the form (value defaults to "1"):