docs/manual: add musl to toolchain dependency options

Currently, we only document (e)glibc and uClibc as C libraries on which
we can depend. However, we've supported musl for quite some time now.

So, also refference it in the manual, when dealing with toolchain
options a package may have to depend on.

[Peter: order musl before uClibc]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>