Some of the C library string functions have an unknown provenance.
Reimplement all such functions to avoid potential licensing
uncertainty.

Remove the inline-assembler versions of strlen(), memswap(), and
strncmp(); these save a minimal amount of space (around 40 bytes in
total) and are not performance-critical.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

Signed-off-by: Michael Brown <mcb30@ipxe.org>

Signed-off-by: Michael Brown <mcb30@ipxe.org>

Add FILE_LICENCE declarations to almost all files that make up the
various standard builds of gPXE.

away code such as

  if ( strcmp ( some_string, "some_value" ) != 0 ) {
     DBG ( "Bad value received!" );
  }