[spi] add spi_init_slaves back and fix makefile for b2

conf/firmwares/subsystems/shared/imu_b2_common.makefile

@@ -46,11 +46,11 @@ imu_srcs += peripherals/max1168.c
 imu_srcs += $(SRC_ARCH)/peripherals/max1168_arch.c
 
 include $(CFG_SHARED)/spi.makefile
-imu.CFLAGS += -DUSE_SPI_SLAVE0
-imu.CFLAGS += -DSPI_MASTER
+imu_CFLAGS += -DUSE_SPI_SLAVE0
+imu_CFLAGS += -DSPI_MASTER
 
 ifeq ($(ARCH), lpc21)
-imu.CFLAGS += -DUSE_SPI1
+imu_CFLAGS += -DUSE_SPI1
 imu_CFLAGS += -DSSP_VIC_SLOT=9
 imu_CFLAGS += -DMAX1168_EOC_VIC_SLOT=8
 else ifeq ($(ARCH), stm32)

sw/airborne/mcu.c

@@ -109,6 +109,9 @@ void mcu_init(void) {
 #if USE_SPI2
   spi2_init();
 #endif
+#if USE_SPI0 || USE_SPI1 || USE_SPI2
+  spi_init_slaves();
+#endif
 #if USE_SPI0_SLAVE
   spi0_slave_init();
 #endif