doc: update reference to README.imx8image

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

README.imx8image  => imx/mkimage/imx8image.txt
Signed-off-by: NPatrick Delaunay <patrick.delaunay@st.com>

board/freescale/imx8qxp_mek/imximage.cfg

@@ -2,7 +2,7 @@
 /*
  * Copyright 2018 NXP
  *
- * Refer doc/README.imx8image for more details about how-to configure
+ * Refer doc/imx/mkimage/imx8image.txt for more details about how-to configure
  * and create imx8image boot image
  */
 

board/siemens/capricorn/imximage.cfg

@@ -2,7 +2,7 @@
 /*
  * Copyright 2018 NXP
  *
- * Refer doc/README.imx8image for more details about how-to configure
+ * Refer doc/imx/mkimage/imx8image.txt for more details about how-to configure
  * and create imx8image boot image
  */
 

board/toradex/apalis-imx8/apalis-imx8qm-imximage.cfg

@@ -2,7 +2,7 @@
 /*
  * Copyright 2019 Toradex
  *
- * Refer doc/README.imx8image for more details about how-to configure
+ * Refer doc/imx/mkimage/imx8image.txt for more details about how-to configure
  * and create imx8image boot image
  */
 

board/toradex/colibri-imx8x/colibri-imx8qxp-imximage.cfg

@@ -2,7 +2,7 @@
 /*
  * Copyright 2019 Toradex
  *
- * Refer doc/README.imx8image for more details about how-to configure
+ * Refer doc/imx/mkimage/imx8image.txt for more details about how-to configure
  * and create imx8image boot image
  */