libcoap: Add recipe

libcoap implements a lightweight application-protocol for devices that
are constrained their resources such as computing power, RF range,
memory, bandwith, or network packet sizes.

Original patch is from Alex Kiernan <alex.kiernan@gmail.com>.
Upstream commit in meta-openembedded:

  1556da89029c364bea101e782bdd016b69cddb82 ("libcoap: Add recipe")

Importing the patch because the recipe is not present in 
kirkstone branch but only in master branch of meta-openembedded.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Alex Kiernan <alexk@zuma.ai>
Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@huawei.com>
Signed-off-by: Francesco Pham <francesco.pham@huawei.com>