exFAT improvements, fixes

Move Exec_Cmd to libcrecovery __popen
Provide opt out build flag for exFAT
Default fstype to exfat on external storage if exfat support is
present and fstype is vfat or auto
Fix invalid unmount errors
Improve handling of unencrypted sdcards on Samsung devices
diff --git a/prebuilt/Android.mk b/prebuilt/Android.mk
index 5737431..5e78b34 100644
--- a/prebuilt/Android.mk
+++ b/prebuilt/Android.mk
@@ -44,10 +44,10 @@
 RELINK_SOURCE_FILES += $(TARGET_OUT_SHARED_LIBRARIES)/libstlport.so
 RELINK_SOURCE_FILES += $(TARGET_OUT_SHARED_LIBRARIES)/libmincrypt.so
 RELINK_SOURCE_FILES += $(TARGET_OUT_SHARED_LIBRARIES)/libext4_utils.so
-RELINK_SOURCE_FILES += $(TARGET_RECOVERY_ROOT_OUT)/sbin/mkexfatfs
 
-ifeq ($(TW_INCLUDE_FUSE_EXFAT), true)
+ifneq ($(TW_NO_EXFAT), true)
     RELINK_SOURCE_FILES += $(TARGET_RECOVERY_ROOT_OUT)/sbin/exfat-fuse
+    RELINK_SOURCE_FILES += $(TARGET_RECOVERY_ROOT_OUT)/sbin/mkexfatfs
 endif
 ifeq ($(TW_INCLUDE_BLOBPACK), true)
     RELINK_SOURCE_FILES += $(TARGET_RECOVERY_ROOT_OUT)/sbin/blobpack