targets/nordic_pca10095-blehci: Enable coredump support

[kasjer]

This enables (by default) coredump generation on net core. It also enables net core flash server that can be used to get coredumps from application core.

[vrahane]

@kasjer img_mgmt.h needs to be included conditionally based on LOG_SOFT_RESET in repos/apache-mynewt-mcumgr/cmd/os_mgmt/port/mynewt/src/mynewt_os_mgmt.c, that's why the build is failing.