fastboot: implement fastbootd in twrp

fastbootd is a userspace daemon that runs in the recovery
ramdisk. It allows writing single partitions to the super
partition via fastboot CLI. When TWRP sees the fastboot option
in the misc partition during startup, it will start with
the fastbootd page and fastbootd running. Otherwise it will
run recovery normally.

Change-Id: Id7b12d29ef21dbd5950dc884dd1cd788f25357f0
diff --git a/variables.h b/variables.h
index 51ee09f..95946b7 100755
--- a/variables.h
+++ b/variables.h
@@ -130,6 +130,7 @@
 #define TW_MIN_SYSTEM_VAR           "tw_min_system"
 #define TW_DOWNLOAD_MODE            "tw_download_mode"
 #define TW_EDL_MODE                 "tw_edl_mode"
+#define TW_FASTBOOT_MODE            "tw_fastboot_mode"
 #define TW_IS_ENCRYPTED             "tw_is_encrypted"
 #define TW_IS_DECRYPTED             "tw_is_decrypted"
 #define TW_CRYPTO_PWTYPE            "tw_crypto_pwtype"