eficonfig: use efi_get_next_variable_name_int()

eficonfig command reads all possible UEFI load options
from 0x0000 to 0xFFFF to construct the menu. This takes too much
time in some environment.
This commit uses efi_get_next_variable_name_int() to read all
existing UEFI load options to significantlly reduce the count of
efi_get_var() call.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>