bootstd: Support for distro specific EFI folders

[Shantur Rathore]

Hi Simon,

Currently bootstd - bootmethod_efi only looks for the default
bootxx64.efi in /EFI/boot folder only.
Generally many distros end up putting their bootloaders in
EFI/<distro> folders like EFI/ubuntu and EFI/debian etc.

In x86 worlds, the NVRAM is modified and new boot entries are added to
support these but in the U-boot world the NVRAM variables are
read-only.

What would be the best way to implement this?

I was thinking of having a "efi_prefixes" environment variable which
can be set to "ubuntu debian centos" etc and bootmethod_efi can try
all of them. Will bootmethod_efi be able to support multiple entries (
thinking of multiboot ) ?

Regards,
Shantur