stm32/boards/ARDUINO_PORTENTA_H7: Revert to single-precision float.

Using repr C is incompatible with double-precision floats on 32-bit arch.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

ports/stm32/boards/ARDUINO_PORTENTA_H7/mpconfigboard.mk

@@ -10,7 +10,7 @@ BOOTLOADER_DFU_USB_PID = 0x035b
 # MCU settings
 MCU_SERIES = h7
 CMSIS_MCU = STM32H747xx
-MICROPY_FLOAT_IMPL = double
+MICROPY_FLOAT_IMPL = single
 AF_FILE = boards/stm32h743_af.csv
 LD_FILES = boards/ARDUINO_PORTENTA_H7/stm32h747.ld
 TEXT0_ADDR = 0x08040000