bootloader_message: Allow TWRP to ignore wipe-data command into misc

* Rebase of https://gerrit.omnirom.org/#/c/20750/ patch-set 3
 * Original commit message:

fix: recently a change in AOSP causes a bootloop to TWRP
(and therefore all ROM's based on it)
Reason: AOSP enables use of /misc partition and normally the fstab in
TWRP will not have /misc added. The problem is now when Android writes
a wipe command to /misc it will force TWRP to boot but TWRP will do
nothing - leaving the cmd in /misc. This results in a bootloop to TWRP.
Solution: I added a new var TW_IGNORE_MISC_WIPE_DATA which can be
set to "true" in Boardconfig.mk of a device requiring this which then
ignores /misc commands like it would be when /misc would be just not in
fstab BUT with this change TWRP clears the bit in /misc so a normal boot
can happen.
Besides that if the device do not have an own recovery key combo and
/misc is enabled we want to open TWRP after walking through the factory
reset screen - without actually doing a factory reset.
kudos to nailyk (and DevUt) for the help with the C++ code!

Change-Id: If6fed5872015f1a66304e3edbdb787c507236f69

bootloader_message/bootloader_message.cpp

@@ -284,6 +284,17 @@ get_args(int *argc, char ***argv) {
             (*argv)[0] = strdup(arg);
             for (*argc = 1; *argc < MAX_ARGS; ++*argc) {
                 if ((arg = strtok(NULL, "\n")) == NULL) break;
+
+// if the device does not have an own recovery key combo we just want to open TWRP after
+// walking through the factory reset screen - without actually doing a factory reset
+#ifdef IGNORE_MISC_WIPE_DATA
+                if (!strcmp(arg, "--wipe_data")) {
+                    (*argv)[*argc] = "";
+                    *argc = *argc -1;
+                    printf("Bootloader arg \"%s\" ignored because TWRP was compiled with TW_IGNORE_MISC_WIPE_DATA\n", arg);
+                    continue;
+                }
+#endif
                 (*argv)[*argc] = strdup(arg);
             }
             printf("Got arguments from boot message\n");