qconfig: Allow searching for CONFIG values

Add basic support for searching for matching of non-matching values.

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: https://source.denx.de/u-boot/custodians/u-boot-dm/-/issues/20
diff --git a/doc/develop/qconfig.rst b/doc/develop/qconfig.rst
index 8efb1eb..123779e 100644
--- a/doc/develop/qconfig.rst
+++ b/doc/develop/qconfig.rst
@@ -85,6 +85,20 @@
     3 matches
     pg_wcom_seli8_defconfig highbank_defconfig pg_wcom_expu1_defconfig
 
+It is also possible to search for particular values. For example, this finds all
+boards with an empty string for `CONFIG_DEFAULT_FDT_FILE`::
+
+    ./tools/qconfig.py -f DEFAULT_FDT_FILE=\"\"
+    1092 matches
+    ...
+
+This finds boards which have a value for SYS_MAXARGS other than 64::
+
+    ./tools/qconfig.py -f ~SYS_MAXARGS=64
+    cfg CONFIG_SYS_MAXARGS
+    281 matches
+    ...
+
 
 Finding implied CONFIGs
 -----------------------