BREAKING: drop all part specific ld files

We were never going to be capable of supporting every single part
variant with their own files, so stop trying.  We've been supporting the
linker script generator for a long time now, so move on from these old
static files.

Breaking: if you were using one of these, and don't wish to use the
linker script generator, you should simply declare a stub linker script,
and include the generic section definitions file, and provide this in
your own application.

Old:
LDSCRIPT = $(OPENCM3_DIR)/lib/stm32/l1/stm32l15xxb.ld

New (linker script generator):

DEVICE=stm32l151cb

New (manual):

* Add file mymemorymap.ld, with contents similar too:
```
MEMORY
{
       rom (rx) : ORIGIN = 0x08000000, LENGTH = 256K
       ram (rwx) : ORIGIN = 0x20000000, LENGTH = 48K
}

/* Include the common ld script. */
INCLUDE cortex-m-generic.ld
```

LDSCRIPT=mymemorymap.ld

See ld/README for more information on using the linker script generator

lib/efm32/gg/libopencm3_efm32gg990f1024.ld

@@ -1,15 +0,0 @@
-/* lengths from d046_efm32gg990_datasheet.pdf table 1.1, offset from
- * d0034_efm32tg_reference_manual.pdf figure 5.2.
- *
- * the origins and memory structure are constant over all giantgeckos, but the
- * MEMORY section requires the use of constants, and has thus to be duplicated
- * over the chip variants.
- * */
-
-MEMORY
-{
-	rom (rx) : ORIGIN = 0, LENGTH = 1024k
-	ram (rwx) : ORIGIN = 0x20000000, LENGTH = 128k
-}
-
-INCLUDE cortex-m-generic.ld;