update buildroot to 2017.02.11

bsp/buildroot-2017.02.11/board/beagleboardx15/readme.txt

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+BeagleBoard X15
+
+Intro
+=====
+This config currently supports the beagleboard x15,
+and generates a barebone image.
+
+The image must be flashed to a SD card to be used.
+
+How to build it
+===============
+
+  $ make beagleboardx15_defconfig
+
+Then you can edit the build options using
+
+  $ make menuconfig
+
+Compile all and build a sdcard image:
+
+  $ make
+
+Result of the build
+-------------------
+
+After building, you should get a tree like this:
+
+  output/images/
+  ├── am57xx-beagle-x15.dtb
+  ├── am57xx-beagle-x15-revb1.dtb
+  ├── boot.vfat
+  ├── MLO
+  ├── rootfs.ext2
+  ├── rootfs.ext4
+  ├── rootfs.tar
+  ├── sdcard.img
+  ├── u-boot.img
+  ├── u-boot-spl.bin
+  └── zImage
+
+How to write the microSD card
+=============================
+
+Once the build process is finished you will have an image called "sdcard.img"
+in the output/images/ directory.
+
+Copy the bootable "sdcard.img" onto an SD card with "dd":
+
+  $ sudo dd if=output/images/sdcard.img of=/dev/sdX