wic: bootimg-efi: Support + symbol in filenames

wic: bootimg-efi: Support + symbol in filenames

Allow the '+' symbol as a valid character in filenames listed in the
IMAGE_EFI_BOOT_FILES variable.

The '+' symbol might be used to support boot counting for boot entries,
as described in the UAPI Boot Loader Specification [1]:

The boot counting data is stored in the name of the boot loader entry.
A boot loader entry file name may contain a plus (+) followed by a
number. This may optionally be followed by a minus (-)
followed by a second number. The dot (.) and file name suffix
(conf or efi) must immediately follow.
Boot counting is enabled for entries which match this pattern.

Example:
IMAGE_EFI_BOOT_FILES:append = " entry.conf;loader/entries/entry+3.conf"

[1] https://uapi-group.org/specifications/specs/boot_loader_specification/#boot-counting
Signed-off-by: Igor Opaniuk <igor.opaniuk@foundries.io>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>