efi_loader: utility function to check the variable name is "Boot####"

Some commands need to enumerate the existing UEFI load
option variable("Boot####"). This commit transfers some code
from cmd/efidebug.c to lib/efi_loder/, then exposes
efi_varname_is_load_option() function to check whether
the UEFI variable name is "Boot####".

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

include/efi_loader.h

@@ -707,6 +707,7 @@ int algo_to_len(const char *algo);
 
 int efi_link_dev(efi_handle_t handle, struct udevice *dev);
 int efi_unlink_dev(efi_handle_t handle);
+bool efi_varname_is_load_option(u16 *var_name16, int *index);
 
 /**
  * efi_size_in_pages() - convert size in bytes to size in pages