Provide storage section for writable flash memory

This may be getting a bit too complex to call "minimal", but to support applications that require some permanent storage there needs to be a section defined for that. This probably requires adding the flash page size into memory.x, then having a section something like

.storage BLOCK(_page_size) (NOLOAD) :
{
    FILL(0xFF)
    *(.storage .storage.*)
    . = ALIGN(_page_size);
} > FLASH

If possible, aligning in between each .storage.* section would be wonderful to allow having separately erasable sections (although, I don't know any way to do so).

(Note, I have only used flash storage on the nrf51, I assume other cortex-m devices behave similarly, but this may not be general enough).

Start by examining the linker configuration in memory.x and how Cortex-M flash layouts are currently defined. Determine whether a page-size-backed .storage section can be added generally, including alignment between storage subsections where possible. Done means applications can place permanent data in the section with flash-page alignment and the approach is suitable beyond the nrf51.