arc: introduce "mdbtrick" target

arc: introduce "mdbtrick" target

MetaWare debugger (MDB) is still used as a primary tool for interaction
with target via JTAG. Moreover some very advanced features are not yet
implemented in GDB for ARC (and not sure if they will be implemnted
sometime soon given complexity and rare need for those features for
common user).

So if we're talking about development process when U-Boot is loaded in
target memory not by low-level boot-loader but manually through JTAG
chances are high developer uses MDB for it.

But MDB doesn't support PIE (position-independent executable) - it will
refuse to even start - that means no chance to load elf contents on
target.
Then the only way to load U-Boot in MDB is to fake it by:
  1. Reset PIE flag in ELF header
     This is simpe - on attempt to open elf MDB checks header and if it
doesn't match its expectation refuces to use provided elf.
  2. Strip all debug information from elf
     If (1) is done then MDB will open elf but on parsing of elf's debug
info it will refuse to process due to debug info it cannot understand
(symbols with PIE relocation).

Even though it could be done manually (I got it documented quite a while
ago here http://www.denx.de/wiki/U-Boot/ARCNotes) having this automated
way is very convenient. User may build U-Boot that will be loaded on
target via MDB saying "make mdbtrick".

Then if we now apply the manipulation MDB will happily start and will
load all required sections into the target.

Indeed there will be no source-level debug info available. But still MDB
will do its work on showing disassembly, global symbols, registers,
accessing low-level debug facilities etc.

As a summary - this is a pretty dirty hack but it simplifies life a lot
for us ARc developers.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Tom Rini <trini@ti.com>
Cc: Wolfgang Denk <wd@denx.de>