…e boot entry in optnum"

This reverts commit 1b30c2b.

This commit was creating duplicated entries when the "Linux" entry was
not already in the BootOrder list, which may happen upon firmware reset.

Seen on Dell PowerEdge R740 when booting with BOOTX64 constantly.

This patch keeps the behaviour previous to commit #1b30c2b by returning
the index of the "Linux" entry.
Then a check is made to find the entry in the current BootOrder:
- if it isn't there, prepend the entry and copy the rest (this enlarges
  the BootOrder array by 1)
- if it's there, prepend the entry and copy all remaining entries

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Example of outputs on a Dell PowerEdge R740:
- 0000 is BOOTX64 entry
- other entries are Dell's default ones (internal, no "Linux" there)

1. Entry not already existing in BootOrder

----
set_boot_order:486: Original nbootorder: 3
Original BootOrder: 0000 0003 0004
 :
add_to_boot_list:578: device path: "HD(1,GPT,99D47E76-590F-48FD-8FD6-0A0CE790D635)/\EFI\redhat\shimx64.efi"
find_boot_option:454: Found boot entry "Boot0005" with label "Red Hat Enterprise Linux" for file "\EFI\redhat\shimx64.efi"
add_to_boot_list:623: New nbootorder: 4
BootOrder: 0005 0000 0003 0004
find_boot_options:937: Found directory named "Dell"
update_boot_order:509: nbootorder: 4
BootOrder: 0005 0000 0003 0004
----

2. Entry not existing at all

----
set_boot_order:486: Original nbootorder: 3
Original BootOrder: 0000 0001 0002
 :
add_to_boot_list:578: device path: "HD(1,GPT,99D47E76-590F-48FD-8FD6-0A0CE790D635)/\EFI\redhat\shimx64.efi"
add_boot_option:245: Creating boot entry "Boot0005" with label "Red Hat Enterprise Linux" for file "\EFI\redhat\shimx64.efi"
add_boot_option:282: nbootorder: 4
BootOrder: 0005 0000 0001 0002
find_boot_options:937: Found directory named "Dell"
update_boot_order:509: nbootorder: 4
BootOrder: 0005 0000 0001 0002
----

3. Entry already existing in BootOrder

----
set_boot_order:486: Original nbootorder: 4
Original BootOrder: 0000 0005 0001 0002
 :
add_to_boot_list:578: device path: "HD(1,GPT,99D47E76-590F-48FD-8FD6-0A0CE790D635)/\EFI\redhat\shimx64.efi"
find_boot_option:454: Found boot entry "Boot0005" with label "Red Hat Enterprise Linux" for file "\EFI\redhat\shimx64.efi"
add_to_boot_list:623: New nbootorder: 4
BootOrder: 0005 0000 0001 0002
find_boot_options:937: Found directory named "Dell"
update_boot_order:509: nbootorder: 4
BootOrder: 0005 0000 0001 0002
----