boards/e104-bt5011a-tb: add support for E104-BT5011A breakout board

This adds support for the nRF52811 based E104-BT5011A break-out board.

boards/e104-bt5011a-tb/Kconfig

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+# Copyright (c) 2020 Benjamin Valentin
+#
+# This file is subject to the terms and conditions of the GNU Lesser
+# General Public License v2.1. See the file LICENSE in the top level
+# directory for more details.
+
+config BOARD
+    default "e104-bt5011a-tb" if BOARD_E104_BT5011A_TB
+
+config BOARD_E104_BT5011A_TB
+    bool
+    default y
+    select BOARD_COMMON_E104_BT50XXA_TB
+    select CPU_MODEL_NRF52811XXAA
+
+source "$(RIOTBOARD)/common/e104-bt50xxa-tb/Kconfig"

boards/e104-bt5011a-tb/Makefile

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+MODULE = board
+
+DIRS = $(RIOTBOARD)/common/e104-bt50xxa-tb
+
+include $(RIOTBASE)/Makefile.base

boards/e104-bt5011a-tb/Makefile.dep

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+USEMODULE += boards_common_e104_bt50xxa_tb
+
+include $(RIOTBOARD)/common/e104-bt50xxa-tb/Makefile.dep

boards/e104-bt5011a-tb/Makefile.features

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+CPU_MODEL = nrf52811xxaa
+
+include $(RIOTBOARD)/common/e104-bt50xxa-tb/Makefile.features

boards/e104-bt5011a-tb/Makefile.include

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+include $(RIOTBOARD)/common/e104-bt50xxa-tb/Makefile.include

boards/e104-bt5011a-tb/doc.txt

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+/**
+@defgroup    boards_e104-bt5011a-tb E104-BT5011A Test Board
+@ingroup     boards
+@brief       Support for the Ebyte E104-BT5011A Test Board (nRF52811)
+
+## General information
+
+The E104-BT5011A Test Board is a cheap break-out board for the E104-BT5011A module.
+It's based on the nRF52811 and by default comes with a Firmware that accepts AT
+commands over UART, but it can be flashed with RIOT easily.
+
+ - [Module Datasheet](http://www.ebyte.com/en/downpdf.aspx?id=1074)
+ - [Module Website](http://www.ebyte.com/en/product-view-news.aspx?id=1074)
+
+## Flashing the board
+
+To flash the board, you have to connect a SWD programmer to the pins labeled SWDIO and
+SWCLK. Also make sure to connect GND.
+P0.21 is Reset.
+
+## Accessing STDIO via UART
+
+The STDIO is directly accessible via the USB port. On a Linux host, it's
+generally mapped to `/dev/ttyUSB0`.
+
+Use the `term` target to connect to the board serial port<br/>
+```
+    make BOARD=e104-bt5011a-tb -C examples/hello-world term
+```
+
+ */