musl: Make only shared libraries conditional

External toolchain use requires a static libc (as buildroot uses
`gcc --print-file-name libc.a` to find the sysroot); thus, the static portion
of a musl build should not be conditional.

Signed-off-by: Charles Duffy <chaduffy@cisco.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>