efibootmgr: Document -e command line switch

-e env will include env in the boot loader. Document that the boot
loader appends the env to the BootXXXX variable, and will parse it as
a series of a=b values to set in the boot loader's environment. These
assignments are separated by spaces. The env arg needs to be quoted if
more than one env var is to be set (we parse only the next argument on
the command line).

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