libcoap is not building in LEDE any more because it is extracted to
/libcoap/ and the package makefiles try to access it at
/libcoap-2da31de732c0e51a9bc9e1d4aea21e25da89cf87/.

This patch defines a PKG_BUILD_DIR variable to make it use /libcoap/
without the hash.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

Without the trailing _S_ the variable is not considered at all.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>

Fixes github issue #1856

Signed-off-by: Karl Palsson <karlp@tweak.net.au>

Signed-off-by: Anton Glukhov <anton.a.glukhov@gmail.com>