How could Ya'akov (who was the epitome of Emes), lie so blatantly to his father?
Rashi: He meant "I am (the one bringing to you), and Esav is your firstborn 1 , it was just a white lie (it was true, but Yitzchak understand differently). 2 Likewise, when he said "Asisi ka'Asher Dibarta Elai", indeed, he had done many things like`Yitzchak said - that too was no more than a white lie.
Chizkuni #1: For a temporary need, one may deviate from the truth, like Avraham said "say that I am your brother."
Chizkuni #2: Yakov is the Bechor, for he bought the Bechorah from Esav.
R. Bechayei: You give the Brachos due to Bechorah. Since I bought the Bechorah from Esav, I am in place of him.
Asarah Ma'amaros Perek 18, #1: Yakov was created from the first drop of semen. 3
Asarah Ma'amaros Perek 18, #2: Yakov was the Bechor to Shamayim (his Neshamah was created first). Esav was the physical Bechor.
Presumably, Rashi learns this from the fact that Ya'akov said "Anochi" and not 'Ani', both here, and in Pasuk 24. Note that (in Pasuk 32), in answer to the same question, Esav replied "Ani Bincha Bechorcha Esav". 2
In fact, Ya'akov would have been permitted to lie directly (bearing in mind that he was fulfilling his mother's express orders and that, as the commentaries explain, he was performing a Mitzvah, by taking away from Esav of which he was not worthy), and it is only because he was the Ish Emes, that he minimized the deviation from the truth in this way.
רש"י: אנכי המביא לך, ועשו הוא בכורך: מנין לפרש כך את דברי יעקב?
גור אריה: אילו שיקר לא היה מתברך, כי הקב"ה שונא השקר.
רש"י: אנכי המביא לך, ועשו הוא בכורך: כיצד זה רמוז בדברי יעקב?
גור אריה: עשיו אמר "אני בנך בכורך עשיו" ואילו יעקב אמר "אנכי עשיו בכורך", ו'אני' בסמיכות מנוקד בשוא [-חטף פתח] וכשאיננו סמוך האל"ף קמוצה, ואז היה יצחק מבין שהוא איננו עשיו, ולכן אמר יעקב מילת 'אנכי' שמנוקדת בכל אופן בקמץ.