Was the build process recently updated to output .elf instead of .img?
I'm on 1.0.0 for everything (tools and core repo).
On OS X with GCC 5.4.1 (the latest release from ARM) I get:
*$ newt run bleuart*
Compiling bin/targets/bleuart/generated/src/bleuart-sysinit-app.c
Archiving bleuart-sysinit-app.a
Linking
/Users/ktown/Dropbox/microBuilder/Code/nRF52/Mynewt/Adafruit_Mynewt/bin/targets/bleuart/app/apps/bleuart/bleuart.elf
Loading app image into slot 1
Error: Cannot find file
/Users/ktown/Dropbox/microBuilder/Code/nRF52/Mynewt/Adafruit_Mynewt/bin/targets/bleuart/app/apps/bleuart/bleuart.img
*$ ls bin/targets/bleuart/app/apps/bleuart/**
*apps/ apps_bleuart.a.cmd bleuart.elf.bin*
bleuart.elf.lst manifest.json
apps_bleuart.a bleuart.elf* bleuart.elf.cmd bleuart.elf.map
*$ arm-none-eabi-gcc --version**
*arm-none-eabi-gcc (GNU Tools for ARM Embedded Processors) 5.4.1
20160919 (release) [ARM/embedded-5-branch revision 240496]
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
