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/ge/bx50v3/bx50v3.cfg b/board/ge/bx50v3/bx50v3.cfg
index f872ea2..de3955a 100644
--- a/board/ge/bx50v3/bx50v3.cfg
+++ b/board/ge/bx50v3/bx50v3.cfg
@@ -4,7 +4,7 @@
  * Copyright 2015 Timesys Corporation.
  * Copyright 2015 General Electric Company
  *
- * 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