Refer to header default pin modes
http://beagleboard.org/static/images/cape-headers.png

P9_17 (SPI0_CSO => CE0) enables peripheral device
P9_18 (SPI0_D1 => MOSI) outputs data to peripheral device
P9_21 (SPIO_DO => MISO) receives data from peripheral device
P9_22 (SPI0_SCLK => SCLK) outputs clock signal

Use config-pin to set pin mode for SPI pins
https://github.com/beagleboard/bb.org-overlays/tree/master/tools/beaglebone-universal-io

config-pin p9.17 spi_cs
config-pin p9.18 spi
config-pin p9.21 spi
config-pin p9.22 spi_sclk

reuse the Raspberry Pi SPI class as both are
Linux boards using spidev.  The main difference
is the pins numbers are different.

Testing with Adafruit_CircuitPython_BME280 in SPI mode.

TODO: wire up the board to the BeagleBone Black

Current output:

debian@beaglebone:~/Adafruit_CircuitPython_BME280$ sudo python3 examples/bme280_simpletest.py
SPI(): __init()
SPI(): beaglebone_black: from adafruit_blinka.microcontroller.raspi_23.spi import SPI as _SPI
spiPorts: ((0, P9_22, P9_18, P9_21), (1, P9_22, P9_18, P9_21))
for:
0 P9_22 P9_18 P9_21
Line 91
<class 'adafruit_blinka.microcontroller.raspi_23.spi.SPI'>
<adafruit_blinka.microcontroller.raspi_23.spi.SPI object at 0xb6b3c890>
Traceback (most recent call last):
  File "examples/bme280_simpletest.py", line 15, in <module>
    bme280 = adafruit_bme280.Adafruit_BME280_SPI(spi, bme_cs)
  File "/usr/local/lib/python3.5/dist-packages/adafruit_circuitpython_bme280-2.1.1.dev3+g1416d93-py3.5.egg/adafruit_bme280.py", line 247, in __init__
  File "/usr/local/lib/python3.5/dist-packages/adafruit_circuitpython_bme280-2.1.1.dev3+g1416d93-py3.5.egg/adafruit_bme280.py", line 78, in __init__
RuntimeError: Failed to find BME280! Chip ID 0x0

Refer to:
http://beagleboard.org/static/images/cape-headers-spi.png

CE1 P9.28 SPI1_CS0
MISO_1 P9.29 SPI1_D0
MOSI_1 P9.30 SPI1_D1
SCLK_1 P9.31 SPI_SCLK

SPI1 conflicts with HDMI Audio (McASP)

Refer to:
https://elinux.org/Beagleboard:BeagleBoneBlack_Debian#U-Boot_Overlays

To Disable HDMI AUDIO, uncomment this line in /boot/uEnv.txt:
disable_uboot_overlay_audio=1

Set pin modes for SPI1 with:

config-pin p9.28 spi_cs
config-pin p9.29 spi
config-pin p9.30 spi
config-pin p9.31 spi_sclk