kernel-fitimage: sanitize dtb section name (unbreak MIPS)

We can't build fitImages for MIPS any more:
| Error: fit-image.its:21.27-28 syntax error
| FATAL ERROR: Unable to parse input tree
| uboot-mkimage: Can't read arch/mips/boot/fitImage.tmp: Invalid argument

Since commit cd2ed7f80b555add07795cc0cbaee866e6c193a3
("kernel-fitimage: dtb sections named by their filenames
and one config section for each dtb"), commit
1ec405ef5d in yocto, dtb
sections are named by the DTB filename, but the filename
can legally be in a subdirectory below
arch/$arch/boot/dts/, and on MIPS all DTBs are actually
in a subdirectory.

If so, mkimage fails with the above error message.

Unbreak this by replacing the offending character
(directory separator /)

(From OE-Core rev: feb78cb536a5e5cc9f1c36014b1a1ae1c147f45c)

Signed-off-by: André Draszik <adraszik@tycoint.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>

(cherry picked from commit 335fc50cf54e47db4e3d5c35a9846484faf0270f)
Signed-off-by: André Draszik <adraszik@tycoint.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/kernel-fitimage.bbclass

@@ -351,6 +351,7 @@ fitimage_assemble() {
 				DTB_PATH="arch/${ARCH}/boot/${DTB}"
 			fi
 
+			DTB=$(echo "${DTB}" | tr '/' '_')
 			DTBS="${DTBS} ${DTB}"
 			fitimage_emit_section_dtb ${1} ${DTB} ${DTB_PATH}
 		done