toolchain/toolchain.mk: fix code style

toolchain/toolchain.mk: fix code style

Rearrange the header of the .mk file so it becomes similar to a header
from a package. It doesn't fit in one line, so split the details to a
comment below the header.

GCONV_LIBS is only used inside this file, so rename it to start with
TOOLCHAIN_, following the namespace convention already used by packages.

Rename the hook COPY_GCONV_LIBS to TOOLCHAIN_GLIBC_COPY_GCONV_LIBS
following the convention used for hooks in packages.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>