fastboot: use constants.h values

Cleanup to utilize all the manifest values in constants.h.

If the cli command _and_ the protocol name match, use a common
source of convenient truth.  This should set a pattern for future
additional commands.  When the command and the protocol differ,
we want to introduce resistance as it results in confusion and
maintenance issues.

Test: compile
Change-Id: Idad413c63cbbfcb6e851856105a5d5a9ef53ef29
diff --git a/fastboot/fastboot_driver.h b/fastboot/fastboot_driver.h
index 62bbe52..af02637 100644
--- a/fastboot/fastboot_driver.h
+++ b/fastboot/fastboot_driver.h
@@ -37,15 +37,14 @@
 #include <bootimg.h>
 #include <inttypes.h>
 #include <sparse/sparse.h>
+
+#include "constants.h"
 #include "transport.h"
 
 class Transport;
 
 namespace fastboot {
 
-static constexpr int FB_COMMAND_SZ = 64;
-static constexpr int FB_RESPONSE_SZ = 64;
-
 enum RetCode : int {
     SUCCESS = 0,
     BAD_ARG,