This allows for APKBUILDs to know the target endianness in the same way as it knows the target $CARCH. In cross-compile situation, specifying $CENDIAN via environment or abuild.conf is necessary.
This will be expanded on in the form of $arch hints, arch="all !big-endian" for example to block the package on big-endian systems.
Something similar is in the works for 32-bit vs 64-bit.
To make use of this feature, apk-tools 2.11 or later must be present. See apk-tools!29 for more details.