This fixes a memory leak in lpc3180_nand_device_command by
reordering the malloc to occur after all parsing has completed.

This eliminates redundant code for parsing and retreiving the bank
specified from a script command argument.  This patch was written to
replace existing functionality; however, the parsing logic can be
updated later to allow flash commands to accept bank names as well as
their numbers.

Passes cmd_ctx into parse_load_image_command_args for reporting the
parsing errors therein.

This helper eliminates significant amount of redundant code in command
handler functions throughout the system.  It wraps the lower-level
parse_* macros to implement a policy for reporting parse errors to the
active command context (cmd_ctx).  If errors do occur, this macro causes
the calling function to abort with the proper return code.