AMD Steppe Eagle: Add 32-bit Fletcher's Checksum computation

The AMD Platform Security Processor (PSP) requires a Fletcher's
Checksum at the end of the PSP directory.  This code implements
a Fletcher's Checksum by reading bytes from stdin and writes the
bytes back to stdout with a checksum inserted into the byte stream
at the appropriate offset.

This utility is used on PSP binaries during coreboot build.

Include a runtime debug option such that the command:

	fletcher --print <file.bin >file_with_cksum.bin

will print out the computed checksum value for debugging.  The
compile-time debug option is retained that allows -DDEBUG to
be added to the compilation line.  This option has the same
effect as "--print".

Change-Id: I506a479d8204ca4f8267d53aa152ac4b473dbc75
Signed-off-by: Bruce Griffith <Bruce.Griffith@se-eng.com>
Reviewed-on: http://review.coreboot.org/6676
Reviewed-by: WANG Siyuan <wangsiyuanbuaa@gmail.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Zheng Bao <zheng.bao@amd.com>