buildman: Add documentation for CONFIG checking

The -K option is not mentioned in the README at present. Add some notes
to describe how this is used.

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

tools/buildman/README

@@ -968,6 +968,43 @@ of the source tree, thus allowing rapid tested evolution of the code.
     SOURCE_DATE_EPOCH=0 ./tools/buildman/buildman -I -P tegra
 
 
+Checking configuration
+======================
+
+A common requirement when converting CONFIG options to Kconfig is to check
+that the effective configuration has not changed due to the conversion.
+Buildman supports this with the -K option, used after a build. This shows
+differences in effective configuration between one commit and the next.
+
+For example:
+
+    $ buildman -b kc4 -sK
+    ...
+    43: Convert CONFIG_SPL_USBETH_SUPPORT to Kconfig
+    arm:
+    + u-boot.cfg: CONFIG_SPL_ENV_SUPPORT=1 CONFIG_SPL_NET_SUPPORT=1
+    + u-boot-spl.cfg: CONFIG_SPL_MMC_SUPPORT=1 CONFIG_SPL_NAND_SUPPORT=1
+    + all: CONFIG_SPL_ENV_SUPPORT=1 CONFIG_SPL_MMC_SUPPORT=1 CONFIG_SPL_NAND_SUPPORT=1 CONFIG_SPL_NET_SUPPORT=1
+    am335x_evm_usbspl :
+    + u-boot.cfg: CONFIG_SPL_ENV_SUPPORT=1 CONFIG_SPL_NET_SUPPORT=1
+    + u-boot-spl.cfg: CONFIG_SPL_MMC_SUPPORT=1 CONFIG_SPL_NAND_SUPPORT=1
+    + all: CONFIG_SPL_ENV_SUPPORT=1 CONFIG_SPL_MMC_SUPPORT=1 CONFIG_SPL_NAND_SUPPORT=1 CONFIG_SPL_NET_SUPPORT=1
+    44: Convert CONFIG_SPL_USB_HOST_SUPPORT to Kconfig
+    ...
+
+This shows that commit 44 enabled three new options for the board
+am335x_evm_usbspl which were not enabled in commit 43. There is also a
+summary for 'arm' showing all the changes detected for that architecture.
+In this case there is only one board with changes, so 'arm' output is the
+same as 'am335x_evm_usbspl'/
+
+The -K option uses the u-boot.cfg, spl/u-boot-spl.cfg and tpl/u-boot-tpl.cfg
+files which are produced by a build. If all you want is to check the
+configuration you can in fact avoid doing a full build, using -D. This tells
+buildman to configuration U-Boot and create the .cfg files, but not actually
+build the source. This is 5-10 times faster than doing a full build.
+
+
 Other options
 =============