Support chained init_boot partition signing

Bug: 256048561
Bug: 256237041
Test: sign_target_files_apks -d certs --avb_init_boot_algorithm \
        SHA256_RSA4096 --avb_init_boot_key init_boot_rsa4096.pem \
	xxx.zip signed.zip (Check signed.zip/META/misc_info.txt)
Change-Id: I65fc7913089ae318d90df55a533d3e2ebd93b029