doc: update reference to README.imximage

Update reference in many files detected by
scripts/documentation-file-ref-check

README.imximage => imx/mkimage/imximage.txt

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/board/boundary/nitrogen6x/nitrogen6dl.cfg b/board/boundary/nitrogen6x/nitrogen6dl.cfg
index 56b3bcb..9558e26 100644
--- a/board/boundary/nitrogen6x/nitrogen6dl.cfg
+++ b/board/boundary/nitrogen6x/nitrogen6dl.cfg
@@ -2,7 +2,7 @@
 /*
  * Copyright (C) 2013 Boundary Devices
  *
- * Refer doc/README.imximage for more details about how-to configure
+ * Refer doc/imx/mkimage/imximage.txt for more details about how-to configure
  * and create imximage boot image
  *
  * The syntax is taken as close as possible with the kwbimage