samples: nvs: set MAIN_STACK_SIZE in prj.conf

- Sets MAIN_STACK_SIZE in prj.conf to 2048 (default is 1024)
  to avoid possible stack overflow
  and adding new .conf files per board.
- Deletes existing board .conf files, as they are not needed anymore.

Signed-off-by: Andrej Butok <andrey.butok@nxp.com>

Author: butok

samples/subsys/nvs/boards/nucleo_f091rc.conf

@@ -6,3 +6,4 @@ CONFIG_LOG_MODE_IMMEDIATE=y
 CONFIG_NVS_LOG_LEVEL_DBG=y
 CONFIG_REBOOT=y
 CONFIG_MPU_ALLOW_FLASH_WRITE=y
+CONFIG_MAIN_STACK_SIZE=2048