android-gadget-setup: fix conditional argument to assign serial variable

Change the conditional argument to check if androidserial is non-empty to an
if condition instead. `[ -n ]` is an incomplete conditional test. In bash and
similar shells, `[ -n ]` without a string following `-n` is considered a syntax
error. /usr/bin/android-gadget-setup starts with `set -e` is used to change
behavior of the shell in various ways. `-e` option causes the shell to exit as
soon a command exits with a non-zero status. The syntax error in `[ -n ]`
causes the script to step and hence, android-tools-adbd service doesn't start.

dynamic-layers/openembedded-layer/recipes-devtools/android-tools/android-tools-conf-configfs/qcom/android-gadget-setup.machine

@@ -1,4 +1,6 @@
 manufacturer=Qualcomm
 model=`hostname`
 androidserial="$(sed -n -e '/androidboot.serialno/  s/.*androidboot.serialno=\([^ ]*\).*/\1/gp ' /proc/cmdline)"
-[ -n "$androidserial" ] && serial="$androidserial"
+if [ -n "$androidserial" ]; then
+serial="$androidserial"
+fi