nrf: expose the RAM base address

The RAM base address is needed during SoftDevice initialization. So far,
the same magic value has been used in aykevl/go-bluetooth and in TinyGo,
but this should be configured in only one place.

This will have additional benefits in the future:

  * It is currently set to 0x39c0, which is around 14.5kB. Most nrf51822
    chips have only 16kB of RAM, so this is way too much for those
    chips.
  * LLD in LLVM 11 allows expressions in the MEMORY part of linker
    scripts, which will allow overriding the SoftDevice RAM area with a
    linker flag, which might come in handy.

targets/nrf52-s132v6.ld

@@ -7,4 +7,7 @@ MEMORY
 
 _stack_size = 4K;
 
+/* This value is needed by the Nordic SoftDevice. */
+__app_ram_base = ORIGIN(RAM);
+
 INCLUDE "targets/arm.ld"

targets/nrf52840-s140v7.ld

@@ -7,4 +7,7 @@ MEMORY
 
 _stack_size = 4K;
 
+/* This value is needed by the Nordic SoftDevice. */
+__app_ram_base = ORIGIN(RAM);
+
 INCLUDE "targets/arm.ld"