libcurl: enable mbedtls support

libcurl: enable mbedtls support

Now that we've got an mbedtls package in the tree we can enable the
optional support for it in libcurl.

We also remove the comment about polarssl support needing version
1.3.x. Indeed, polarssl was renamed to mbedtls when bought by ARM,
which was circa the 1.3.x polarssl release. Due to this referring to
polarssl 1.3.x doesn't make a lot of sense, and we'll probably never
package polarssl 1.3.x in Buildroot now that mbedtls replaces it.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Tested-by: Luca Ceresoli <luca@lucaceresoli.net>
[Thomas: slightly improve commit log as suggested by Luca, using
explanations from Gustavo.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>