Wait for device to disappear after reboot-bootloader.

(Linux only for now) With fastboot reading serial numbers from sysfs, it had
become possible for a fastboot command issued immediately after rebooting
the bootloader to fail, because sysfs still thought the device was online.
To prevent this, after reboot-bootloader we wait for the device to disconnect.

Also made usb_read and usb_write fail immediately if the descriptor has been
closed; this prevents an incorrect error message ("Bad file descriptor")
when errors from fb_getvar are ignored (e.g., by fb_format_supported).

Also removed unused fd param from filter_usb_device, and simplified logic
in usb_write by using do/while instead of a special case for len == 0.

Change-Id: I799b857eab411fd8ad25f5777fc61c685152ea86
diff --git a/fastboot/fastboot.c b/fastboot/fastboot.c
index 447b257..70b838f 100644
--- a/fastboot/fastboot.c
+++ b/fastboot/fastboot.c
@@ -30,7 +30,6 @@
 
 #include <stdio.h>
 #include <stdlib.h>
-#include <stdarg.h>
 #include <stdbool.h>
 #include <stdint.h>
 #include <string.h>
@@ -82,17 +81,6 @@
 unsigned tags_offset    = 0x00000100;
 
 
-void die(const char *fmt, ...)
-{
-    va_list ap;
-    va_start(ap, fmt);
-    fprintf(stderr,"error: ");
-    vfprintf(stderr, fmt, ap);
-    fprintf(stderr,"\n");
-    va_end(ap);
-    exit(1);
-}
-
 void get_my_path(char *path);
 
 char *find_item(const char *item, const char *product)
@@ -1031,6 +1019,7 @@
         fb_queue_reboot();
     } else if (wants_reboot_bootloader) {
         fb_queue_command("reboot-bootloader", "rebooting into bootloader");
+        fb_queue_wait_for_disconnect();
     }
 
     if (fb_queue_is_empty())