buildroot

Remove TOOLCHAIN_DIR

The $(O)/toolchain directory, also called $(TOOLCHAIN_DIR) is no
longer needed, as all packages are now built in $(O)/build/, including
gcc and uClibc.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>