apps/netutils/tftpc: This commit modifies the TFTP client functions to use a...

apps/netutils/tftpc:  This commit modifies the TFTP client functions to use a data read/write callback instead of a file.This allows TFTP to write to arbitrary destination (in my case, a MTD device - for firmware update).  Two new functions are introduced for this, named tftpget_cb and tftpput_cb. They are just made of most of the existing code.  The previously existing tftpget/tftpput functions are now wrappers on the new ones, with callbacks that read/write from files, so my modifications are backwards compatible with existing applications, eg the associated nsh commands dont need to be changed.