meta-toolchain: make SDK relocatable by using $SDK_PATH var in env setup script

meta-toolchain: make SDK relocatable by using $SDK_PATH var in env setup script

Do not hard-code SDK location into all the libtool's .la files and other
libtool, pkg-config and opkg service variables and aliases. Use $SDK_PATH
environment variable instead, which is set once in the main environment-setup
script, allowing easy SDK relocation by adjusting a single variable.

This patch does not address gcc relocatability, if any.

Signed-off-by: Denys Dmytriyenko <denis@denix.org>