Extract space trimming code from ATA and use in USB and bootorder code.

Introduce function nullTrailingSpace() that nulls blank characters
from the end of a string.  Use this function in the ATA, USB MSC, and
bootorder code.
diff --git a/src/boot.c b/src/boot.c
index e5475a2..b59794e 100644
--- a/src/boot.c
+++ b/src/boot.c
@@ -51,6 +51,7 @@
         f = strchr(f, '\n');
         if (f)
             *(f++) = '\0';
+        nullTrailingSpace(Bootorder[i]);
         dprintf(3, "%d: %s\n", i+1, Bootorder[i]);
     } while (f);