script: Make the get_default_envs.sh script working with newest u-boot

This commit fixes several issues:

- After moving env related code to ./env directory the env_common.o file
is no longer present in the system (has been replaced with built-in.o).

- Use ${OBJCOPY} if available, fallback to system default's objcopy if not
present.

- Extend the script to accept different build directory than current one.
It is extremely handy with OE usage, where source code is separated from
build.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Tested-by: Alex Kiernan <alex.kiernan@gmail.com>