I'm not the original poster, but starting with bare MCU might be too tricky at first. A better approach imho would be to get some of many available kits (i.e. Beaglebone) and try to make it live from scratch: don't use prepared bootloader/OS, but try to build everything yourself. Setting up a toolchain, grasping cross-compiling, downloading and debugging "doesn't start" kind of problems are the key and every day problems in embedded programing. When you are familiar with the build-download-debug process, get into peripherals: GPIO, UART, SPI, memory interfaces etc. After this, you should have enough experience and spent enough time on reading obscure datasheets to start designing your first, simple boards. Sure, you also need some electronics knowledge to read, understand and layout schematics, but provided you don't get into RF or analog electronics, it's not too difficult.
I'm going to disagree here about the Beaglebone (or RPi) first. There is a LOT of complexity in those that simply isn't in the Cortex-M (especially M0) series. For example, try to figure out what the maximum bit banging rate is on a Beaglebone Black. Good luck.
Getting up and running on an Eclipse/ARM/Segger toolchain sucks. No doubt. But you have to chew through it.
After that, it's about 3 lines to get an LED to blink.
