[FS#1285] libcurl with mbedtls async interface doesn't work

[LEDE Bugs]

The following task has a new comment added:

FS#1285 - libcurl with mbedtls async interface doesn't work
User who did this - Darren Tucker (dtucker)

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Cherry-pick change from curl 7.53.0 into lede-17.01 branch: https://github.com/openwrt/openwrt/pull/674
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More information can be found at the following URL:
https://bugs.openwrt.org/index.php?do=details&task_id=1285#comment4143