Fix aosp_x86_64-user build failure

Commit Ia982eb2ee3d1eb64db72c1836e433bcc53e71e3f removes boot-5.4.img
and leaves only a boot-5.10.img file, which makes 'boot_container'
become false. This leads to the failure in AddVbmetaDigest() as it will
search a 'boot.img' based on the 'boot' descriptor from the vbmeta.img.

Add a condition that if boot_images[0] is not 'boot.img' then the
build is also a boot container.

Bug: 199807830
Test: build aosp_x86_64-user
Change-Id: I4a9487b075186f0abf2ba74d3a1cf78072352a05