net: lwip: add support for built-in root certificates

Introduce Kconfig symbols WGET_BUILTIN_CACERT and
WGET_BUILTIN_CACERT_PATH to provide root certificates at build time.

Usage example:

 wget -O cacert.crt https://cacerts.digicert.com/DigiCertTLSECCP384RootG5.crt
 make qemu_arm64_lwip_defconfig
 echo CONFIG_WGET_BUILTIN_CACERT=y >>.config
 echo CONFIG_WGET_BUILTIN_CACERT_PATH=cacert.crt >>.config
 make olddefconfig
 make -j$(nproc) CROSS_COMPILE="ccache aarch64-linux-gnu-"
 qemu-system-aarch64 -M virt -nographic -cpu max \
    -object rng-random,id=rng0,filename=/dev/urandom \
    -device virtio-rng-pci,rng=rng0 -bios u-boot.bin
 => dhcp
 # HTTPS transfer using the builtin CA certificates
 => wget https://digicert-tls-ecc-p384-root-g5.chain-demos.digicert.com/
 1867 bytes transferred in 1 ms (1.8 MiB/s)
 Bytes transferred = 1867 (74b hex)

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
4 files changed