grpc: Split unreferenced ALTS libs to grpc-alts

Package libgrpc++_alts.so.* and libgrpc++_error_details.so.* into a
separate ${PN}-alts subpackage via PACKAGE_BEFORE_PN. No binary or
shared library in gBMC links against these libraries (0 DT_NEEDED
references across the rootfs), avoiding 132 KiB (aarch64) / 26.6 KiB
(armv7a) of unused shared libraries in the target image.

Tested:
Built obmc-phosphor-image for arm32 and arm64. Verified that
libgrpc++_alts.so.* and libgrpc++_error_details.so.* are omitted from
the rootfs and that readelf -d confirms 0 missing DT_NEEDED
dependencies across all rootfs binaries.
- platform5: http://fusion2/f52e85e0-ffd0-3e90-b269-f29c0efed958
- platform15: http://fusion2/13f4d265-fcd4-3c71-9393-96d7eb97061b
- platform18: http://fusion2/27afd1d3-96e1-3b13-8ec6-0df984230234
- platform17: http://fusion2/cc4480a5-ba5e-3937-acea-2416b0fe95ef
- platform11+ext: http://fusion2/f5f759b0-82dd-3708-a13d-a4f4990de224
- platform11: http://fusion2/21ec7f63-ac7d-3a71-b5fd-d1a3f0e37610
- platform25: http://fusion2/b9079702-6179-3eba-afe3-7eb161817d45
Google-Bug-Id: 481049523
Change-Id: I8fb5952ef751f22203c005f849e3713767ad09ac
Signed-off-by: Willy Tu <wltu@google.com>