fs: fat: create correct short names

The current function set_name() used to create short names has the
following deficiencies resolved by this patch:

* Long names (e.g. FOO.TXT) are stored even if a short name is enough.
* Short names with spaces are created, e.g. "A     ~1.TXT".
* Short names with illegal characters are created, e.g. "FOO++BAR".
* Debug output does not not consider that the short file name has no
  concluding '\0'.

The solution for the following bug is split of into a separate patch:

* Short file names must be unique.

This patch only provides the loop over possible short file names.

Fixes: c30a15e590c ("FAT: Add FAT write feature")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/lib/Kconfig b/lib/Kconfig
index 7673d2e..06eb8d0 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -168,7 +168,7 @@
 choice
 	prompt "Pseudo-random library support type"
 	depends on NET_RANDOM_ETHADDR || RANDOM_UUID || CMD_UUID || \
-		   RNG_SANDBOX || UT_LIB && AES
+		   RNG_SANDBOX || UT_LIB && AES || FAT_WRITE
 	default LIB_RAND
 	help
 	  Select the library to provide pseudo-random number generator