image.bbclass: Correct chaining compression support

When chaining of compression/conversion types was added, we had a new
way to handle doing things like "ext4.bz2.sha256sum" or
"ext2.gz.u-boot".  However, because the U-Boot image class isn't
included normally, it wasn't properly converted at the time.  After the
support was added the "clean" argument that the .u-boot code uses no
longer functions.  The fix for this inadvertently broke chaining
compression/conversion.  First, correct the u-boot conversion code.

Fixes: 46bc438374de ("image.bbclass: do exact match for rootfs type")
Cc: Zhenhua Luo <zhenhua.luo@nxp.com>
Cc: Richard Purdie <richard.purdie@linuxfoundation.org>
Cc: Patrick Ohly <patrick.ohly@intel.com>
(From OE-Core rev: e1546856890a438b5ea3763af3cdc13db8e8f236)

Signed-off-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 0a7ce0b971a208956cb895ba5a869ec8c5d94703)

excluded lz4 and lzo chnages
Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/image.bbclass

@@ -457,7 +457,7 @@ python () {
         rm_tmp_images = set()
         def gen_conversion_cmds(bt):
             for ctype in ctypes:
-                if bt[bt.find('.') + 1:] == ctype:
+                if bt.endswith("." + ctype):
                     type = bt[0:-len(ctype) - 1]
                     if type.startswith("debugfs_"):
                         type = type[8:]