file: /proc/sys/kernel/bootloader_type
variable: kernel.bootloader_type
Official reference

x86 bootloader identification

This gives the bootloader type number as indicated by the bootloader, shifted left by 4, and OR’d with the low four bits of the bootloader version. The reason for this encoding is that this used to match the type_of_loader field in the kernel header; the encoding is kept for backwards compatibility. That is, if the full bootloader type number is 0x15 and the full version number is 0x234, this file will contain the value 340 = 0x154.

See the type_of_loader and ext_loader_type fields in Documentation/x86/boot.txt for additional information.