Avoid packing ramdisk into GKI boot-*.img

If init_boot.img is present, the GKI boot-*.img should
not include the ramdisk because the ramdisk will be packed
into the init_boot.img instead.

The "has_ramdisk" flag incorrectly checks the condition of:
prebuilt_name != "boot.img" to see if it is a boot.img.
Because "has_ramdisk" was added before we packed multiple
GKI boot-*.img, e.g., boot-5.10.img, boot-5.10-lz4.img, etc.,
into the target files.

Fix this by checking the partition_name is "boot" or not.
Also moving the logic into a new function with comments for each
condition.

Bug: 203698939
Bug: 213028932
Test: sign_target_files_apks \
        --gki_signing_key=external/avb/test/data/testkey_rsa4096.pem \
        --gki_signing_algorithm=SHA256_RSA4096 \
        ./out/dist/*-target_files-eng.*.zip signed.zip, then
        unpack_bootimg to checks the signed boot-*.img has no ramdisk
Change-Id: I5354669feb54d547dbe797e51b1b1baa187cb7cf