add const keyword to some APIs

Add 'const' keyword to 'char *' parameters to allow command handlers to
pass constant string arguments.  These changes allow the 'args' command
handler to be changed to 'const' in a subsequent patch.

src/pld/xilinx_bit.h

@@ -33,6 +33,6 @@ typedef struct xilinx_bit_file_s
 	uint8_t *data;
 } xilinx_bit_file_t;
 
-int xilinx_read_bit_file(xilinx_bit_file_t *bit_file, char *filename);
+int xilinx_read_bit_file(xilinx_bit_file_t *bit_file, const char *filename);
 
 #endif /* XILINX_BIT_H */