trenz_tec0117: Avoid forcing CPU type (only force to lite variant when VexRiscv is selected=default).

- main_ram mem_map is now directly used by add_sdram when origin is None.
- max_sdram_size/min_l2_data_width are no longer exposed as targets arguments this can
still be used enforced directly in the few cases it is useful.

boards: Add Vendor prefix to platforms/targets name when useful and when multiple boards from the same vendor. (With Retro-Compat on the imports).

./tec0117.py --build --load

Still some FIXMEs but validate use of the embedded SDRAM with LiteDRAM/LiteX:

        __   _ __      _  __
       / /  (_) /____ | |/_/
      / /__/ / __/ -_)>  <
     /____/_/\__/\__/_/|_|
   Build your hardware, easily!

 (c) Copyright 2012-2020 Enjoy-Digital
 (c) Copyright 2007-2015 M-Labs

 BIOS built on Feb  1 2021 13:09:35
 BIOS CRC passed (5abceb2e)

 Migen git sha1: 40b1092
 LiteX git sha1: f324f953

--=============== SoC ==================--
CPU:		VexRiscv_Lite @ 25MHz
BUS:		WISHBONE 32-bit @ 4GiB
CSR:		32-bit data
ROM:		24KiB
SRAM:		4KiB
L2:		0KiB
SDRAM:		8192KiB 16-bit @ 25MT/s (CL-2 CWL-2)

--========== Initialization ============--
Initializing SDRAM @0x40000000...
Switching SDRAM to software control.
Switching SDRAM to hardware control.
Memtest at 0x40000000 (2MiB)...
  Write: 0x40000000-0x40200000 2MiB
   Read: 0x40000000-0x40200000 2MiB
Memtest OK
Memspeed at 0x40000000 (2MiB)...
  Write speed: 5MiB/s
   Read speed: 6MiB/s

--============== Boot ==================--
Booting from serial...
Press Q or ESC to abort boot completely.
sL5DdSMmkekro
Timeout
No boot medium found

--============= Console ================--

litex> mem_list

Available memory regions:
ROM       0x00000000 0x6000
SRAM      0x01000000 0x1000
SPIFLASH  0x80000000 0x1000000
MAIN_RAM  0x40000000 0x800000
CSR       0x82000000 0x10000

litex> mem_test 0x40000000 0x800000

Memtest at 0x40000000 (8MiB)...
  Write: 0x40000000-0x40800000 8MiB
   Read: 0x40000000-0x40800000 8MiB
Memtest OK

litex>

tec0117: disable BIOS XIP from SPI Flash for now since not working (SPÏ Flash set to power down mode with bitstream?).

Still a WIP but able to do the P&R with modifications on LiteX to generate
the IO_PORT constraints but not the IO_LOC for the SDRAM pins.