boards.cfg: update local documentation

The "options" field at the moment is only documented in the changelog
which isn't terribly useful to people without git.
Signed-off-by: NMike Frysinger <vapier@gentoo.org>

boards.cfg

@@ -3,9 +3,7 @@
 #
 # Syntax:
 #	white-space separated list of entries;
-#	each entry has the following fields:
-#
-#	Targetname  Architecture  CPU  Boardname  Vendor  SoC
+#	each entry has the fields documented below.
 #
 #	Unused fields can be specified as "-", or omitted if they
 #	are the last field on the line.
@@ -13,6 +11,16 @@
 #	Lines starting with '#' are comments.
 #	Blank lines are ignored.
 #
+#	The options field takes the form:
+#		<board config name>[:comma separated config options]
+#	Each config option has the form (value defaults to "1"):
+#		option[=value]
+#	So if you have:
+#		FOO:HAS_BAR,BAZ=64
+#	The file include/configs/FOO.h will be used, and these defines created:
+#		#define CONFIG_HAS_BAR	1
+#		#define CONFIG_BAZ	64
+#
 # To keep the list sorted, use something like
 #
 #	:.,$! sort -f -k2,2 -k3,3 -k6,6 -k5,5 -k1,1