cmd_fdt.c: fix parse of byte streams and strings

Commit 4abd844d extended the fdt command parser to handle property
strings which are split across multiple arguments but it was broken for
byte streams and strings.

Byte stream parsing:

 * Fixes where it would terminate early or go into an endless loop.

 * Fixes a 0x00 being inserted into the data if there is a space after
   '[' or a separate argument.

 * Fixes dereferencing the argument pointer after the last argument.

 * Checks for bad characters.

String parsing:

 * Treat multiple arguments as a string list.  This fixes an issue where
   only the last argument was stored.

Signed-off-by: Ken MacLeod <ken@bitsko.slc.ut.us>