Improve configuration of FPGA subsystem

This patch removes the FPGA subsystem configuration through
the CONFIG_FPGA bitmask configuration option.

See README for the new options:

	CONFIG_FPGA,
	CONFIG_FPGA_<vendor>,
	CONFIG_FPGA_<family>

Signed-off-by: Matthias Fuchs <matthias.fuchs@esd-electronics.com>
diff --git a/common/cmd_fpga.c b/common/cmd_fpga.c
index 377a692..fb24395 100644
--- a/common/cmd_fpga.c
+++ b/common/cmd_fpga.c
@@ -58,7 +58,7 @@
 /* Convert bitstream data and load into the fpga */
 int fpga_loadbitstream(unsigned long dev, char* fpgadata, size_t size)
 {
-#if (CONFIG_FPGA & CFG_FPGA_XILINX)
+#if defined(CONFIG_FPGA_XILINX)
 	unsigned int length;
 	unsigned char* swapdata;
 	unsigned int swapsize;