Allow bypassing ASN.1 processing of public key for ED25519

[de-nordic]

Add conditional compilation of ASN.1 decoding of ED25519 key.

This allows to cut out the ASN.1 encoding, which results in reduced flash size.

Comparison on nrf52840dk/nrf52840 build of MCUboot with the ED25519 enabled

west build -p -d builds/nrf52_mcuboot_ed25519_no_asn -b nrf52840dk/nrf52840 bootloader/mcuboot/boot/zephyr/ -DCONFIG_BOOT_SIGNATURE_TYPE_ED25519=y

Reduces code from 40422 bytes to 39918 bytes, when -DCONFIG_BOOT_KEY_IMPORT_BYPASS_ASN=y is added.

Another benefit of the option is that it is no longer needed to have portion of the mbedTLS, that does the ASN.1 support, enabled when compiling MCUboot with ED25519 for TinyCrypt.