expand the BDB acronym in a few key places

For people not readily familiar with the BDB acronym, expand it in
a few README files and usage strings.

BUG=None
TEST=looked at help output
BRANCH=None

Change-Id: I2523e1a289815dd0eaea093c086bc75093579808
Reviewed-on: https://chromium-review.googlesource.com/414244
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Reviewed-by: Vadim Bendebury <vbendeb@chromium.org>

bdb/README

@@ -1,4 +1,4 @@
-BDB library and utilities
+Boot Descriptor Block (BDB) library and utilities
 
 Building:
 ---------

firmware/README

@@ -1,6 +1,10 @@
 
 Here's what's what in the firmware/ directory.
 
+bdb/
+
+  Code for managing Boot Descriptor Blocks (BDB).
+
 include/
 lib/
 

firmware/bdb/README

@@ -1,4 +1,4 @@
-BDB library and utilities
+Boot Descriptor Block (BDB) library and utilities
 
 Building:
 ---------

futility/cmd_bdb.c

@@ -518,6 +518,8 @@ exit:
 static void print_help(int argc, char *argv[])
 {
 	printf("\nUsage: " MYNAME " %s <--create|--add|--resign|--verify>\n"
+	       "\n"
+	       "Utility for managing boot descriptor blocks (BDBs).\n"
 	       "\n"
 	       "For '--add <bdb_file> [OPTIONS]', required OPTIONS are:\n"
 	       "  --data <file>               Data to be added\n"

futility/cmd_show.c

@@ -394,7 +394,7 @@ static const char usage[] = "\n"
 	"\n"
 	"Where FILE could be\n"
 	"\n"
-	"  a BDB\n"
+	"  a boot descriptor block (BDB)\n"
 	"  a keyblock (.keyblock)\n"
 	"  a firmware preamble signature (VBLOCK_A/B)\n"
 	"  a firmware image (bios.bin)\n"