Change usb read serial function

[byl21]

From serial.c :

• The usb_bulk_msg function is called to read data from the USB device's serial port.

• The function checks whether the input buffer contains the string starting with "RES GET_LDR".

• If the correct string is found, the value X is extracted, converted to an integer and returned.

• The reading is repeated up to 10 times if necessary.

• The LDR_value value is updated with the value read and printed using printk.

• This way, the usb_read_serial function is complete and should update the LDR_value variable as needed.

From smartlamp.ino :

The instruction

Serial.printf("RES GET_LDR %d\n", ldrGetValue())

was formatted, which prints a formatted message on the serial port in the following format: RES GET_LDR [LDR value], where [LDR value] is the current value of the light sensor returned by the ldrGetValue() function. Formatting which allowed the driver to receive the data correctly.