Implement 'fastboot format' command

Some filesystems (e.g. ext4) require flushing an initial
fs image, right after erasing it the partition is unusable.

Doing erase,flush emptyfs is a little bit scaring so we have a
separate command that performs it as atomic step:

 - get size of partition
 - create an empty filesystem image
 - erase the partition
 - flush empty fs to the partition

This command applicable only for ext4 filesystem and checks the
partition type before formatting it.

Change-Id: I8529bc3485739487f0d91312f7c0ab1a6e5d8b44
diff --git a/fastboot/fastboot.h b/fastboot/fastboot.h
index 9e043fe..c2f97a2 100644
--- a/fastboot/fastboot.h
+++ b/fastboot/fastboot.h
@@ -43,6 +43,7 @@
 /* engine.c - high level command queue engine */
 void fb_queue_flash(const char *ptn, void *data, unsigned sz);;
 void fb_queue_erase(const char *ptn);
+void fb_queue_format(const char *ptn);
 void fb_queue_require(const char *prod, const char *var, int invert,
         unsigned nvalues, const char **value);
 void fb_queue_display(const char *var, const char *prettyname);