Some more Kobo startup script tweaks (#6147)

* Simplify env siphoning some more

* Nope, shellcheck, quoting this breaks it.

(it preserves the LNs instead of abusing them as free field separators with word splitting).

And, while this does break on env vars with spaces (since they'll be
unquoted), that was already the case of the xargs approach.
If we had read -d & printf %q, we could go with a pure shell approach,
but we don't ;).

* Document a working quoted variant.

For science!

platform/kobo/koreader.sh

@@ -100,8 +100,10 @@ if [ "${VIA_NICKEL}" = "true" ]; then
     # If we were spawned outside of Nickel, we'll need a few extra bits from its own env...
     if [ "${FROM_NICKEL}" = "false" ]; then
         # Siphon a few things from nickel's env (namely, stuff exported by rcS *after* on-animator.sh has been launched)...
-        eval "$(xargs -n 1 -0 <"/proc/$(pidof -s nickel)/environ" | grep -s -F -e DBUS_SESSION_BUS_ADDRESS -e NICKEL_HOME -e WIFI_MODULE -e LANG -e WIFI_MODULE_PATH -e INTERFACE)"
+        # shellcheck disable=SC2046
-        export DBUS_SESSION_BUS_ADDRESS NICKEL_HOME WIFI_MODULE LANG WIFI_MODULE_PATH INTERFACE
+        export $(grep -s -E -e '^(DBUS_SESSION_BUS_ADDRESS|NICKEL_HOME|WIFI_MODULE|LANG|WIFI_MODULE_PATH|INTERFACE)=' "/proc/$(pidof -s nickel)/environ")
+        # NOTE: Quoted variant, w/ the busybox RS quirk (c.f., https://unix.stackexchange.com/a/125146):
+        #eval "$(awk -v 'RS="\0"' '/^(DBUS_SESSION_BUS_ADDRESS|NICKEL_HOME|WIFI_MODULE|LANG|WIFI_MODULE_PATH|INTERFACE)=/{gsub("\047", "\047\\\047\047"); print "export \047" $0 "\047"}' "/proc/$(pidof -s nickel)/environ")"
     fi
 
     # Flush disks, might help avoid trashing nickel's DB...
@@ -120,8 +122,8 @@ fi
 
 # check whether PLATFORM & PRODUCT have a value assigned by rcS
 if [ -z "${PRODUCT}" ]; then
-    eval "$(xargs -n 1 -0 <"/proc/$(pidof -s udevd)/environ" | grep -s -F -e PRODUCT)"
+    # shellcheck disable=SC2046
-    export PRODUCT
+    export $(grep -s -e '^PRODUCT=' "/proc/$(pidof -s udevd)/environ")
 fi
 
 if [ -z "${PRODUCT}" ]; then
@@ -131,8 +133,8 @@ fi
 
 # PLATFORM is used in koreader for the path to the WiFi drivers (as well as when restarting nickel)
 if [ -z "${PLATFORM}" ]; then
-    eval "$(xargs -n 1 -0 <"/proc/$(pidof -s udevd)/environ" | grep -s -F -e PLATFORM)"
+    # shellcheck disable=SC2046
-    export PLATFORM
+    export $(grep -s -e '^PLATFORM=' "/proc/$(pidof -s udevd)/environ")
 fi
 
 if [ -z "${PLATFORM}" ]; then