rpi-cmdline: do_compile: Use pure Python syntax to get CMDLINE

Otherwise the shell snippet fails with `bad syntax` when `CMDLINE`
contains special characters like `${...}` (useful to insert a U-Boot
variable in `cmdline.txt`).

Signed-off-by: Lluis Campos <lluis.campos@northern.tech>

recipes-bsp/bootfiles/rpi-cmdline.bb

@@ -62,7 +62,7 @@ CMDLINE = " \
     "
 
 do_compile() {
-    echo "${@' '.join('${CMDLINE}'.split())}" > "${WORKDIR}/cmdline.txt"
+    echo "${@' '.join(d.getVar('CMDLINE').split())}" > "${WORKDIR}/cmdline.txt"
 }
 
 do_deploy() {