ld scripts: drop duplication of standard sections

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.

lib/lm4f/libopencm3_lm4f.ld

@@ -1,2 +0,0 @@
-/* Yes, we can simply use the lm3s linker script */
-INCLUDE "libopencm3_lm3s.ld"