suggest using lowest end of series for builtin targets

[nerdralph]

For the STM32 builtin targets, I noticed many of the targets are at the high end with lots of flash and RAM. I think it would be more useful to have builtin support for the STM32F103C8 instead of the F103RC. And since the flash algorithm is the same for devices in the same series, being able to specify the target family (i.e. STM32F103) and over-ride the default values for SRAM, total flash, and page size would eliminate the need to download device packs for many MCUs.