Makefile: drop redundant shell call when deriving BASE_DIR

Makefile: drop redundant shell call when deriving BASE_DIR

Commit 173135df5b69dfd5ae6fe6cf2de8833c6f74c143 ("core: re-enter make if
$(CURDIR) or $(O) are not canonical paths") introduced the CANONICAL_O
variable, defined as:

CANONICAL_O := $(shell mkdir -p $(O) >/dev/null 2>&1)$(realpath $(O))

This duplicates the definition of BASE_DIR, by different means:

BASE_DIR := $(shell mkdir -p $(O) && cd $(O) >/dev/null && pwd)

So one of these shell calls is redundant. CANONICAL_O is defined first,
so this commit replaces the BASE_DIR derivation with $(CANONICAL_O).

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>