The leading - makes it rather inconsistent with the majority of other
projects around the world. Use the form everyone else uses.
To solve this, properly pass prefix to inner makes as was always
intended.
Fixes: https://github.com/libopencm3/libopencm3/issues/1058
Instead of every "simple" target having their own duplicate file with
all the section mappings, just provide a single, simple,
"cortex-m-generic.ld" that works with our startup code and any simple
rom/ram system. This also drops the pointless copying of files all over
the place. Using -L flags properly is sufficient, and the standard file
is now in the root of the library already.
