bitbake.conf: re-order DEBUG_PREFIX_MAP

bitbake.conf: re-order DEBUG_PREFIX_MAP

When specifying several -fdebug-prefix-map command line options to
gcc, they are evaluated right to left (last one first).
Normally, the order is irrelevant. However, when we try to map both:

recipe-sysroot-native
recipe-sysroot

the order matters. With the original order most of our debug packages contain
incorrect debug symbols.
Take for example /usr/bin/.debug/setfacl from acl-dbg:

$ strings setfacl | grep native
...
-native/usr/lib/x86_64-poky-linux/gcc/x86_64-poky-linux/7.3.0/include
-native/usr/lib/x86_64-poky-linux/gcc/x86_64-poky-linux/7.3.0/include
-native/usr/lib/x86_64-poky-linux/gcc/x86_64-poky-linux/7.3.0/include
-native/usr/lib/x86_64-poky-linux/gcc/x86_64-poky-linux/7.3.0/include
-native/usr/lib/x86_64-poky-linux/gcc/x86_64-poky-linux/7.3.0/include
-native/usr/lib/x86_64-poky-linux/gcc/x86_64-poky-linux/7.3.0/include
-native/usr/lib/x86_64-poky-linux/gcc/x86_64-poky-linux/7.3.0/include
-native/usr/lib/x86_64-poky-linux/gcc/x86_64-poky-linux/7.3.0/include

Reordering the fdebug-prefix-map arguments will give us the correct paths:
/usr/lib/x86_64-poky-linux/gcc/x86_64-poky-linux/7.3.0/include

Note there may be additional an incorrect paths, such as:
/usr/src/debug/glibc/2.27-r0/recipe-sysroot-native/usr/lib/x86_64-poky-linux.gcc-cross-initial-x86_64/gcc/x86_64-poky-linux/7.3.0/include

But that one needs to be fixed in the recipe for glibc.

Signed-off-by: Juro Bystricky <juro.bystricky@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>