uboot: Use "mdbtrick" target when building elf for ARC

uboot: Use "mdbtrick" target when building elf for ARC

By default position independent executables (PIE) are generated
when building U-Boot elf target. MetaWare debugger doesn't support
PIE (position-independent executable).

In order to allow MDB load and run U-Boot elf, we need to do a
special trick, called "mdbtrick".

This special build target is used to fake generated U-Boot elf by:
    1. Reset PIE flag in ELF header
    2. Strip all debug information from elf

Signed-off-by: Zakharov Vlad <vzakhar@synopsys.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>